Fall back to GLES2 if GLES3 is not working

This adds a static is_viable() method to all rasterizers which has to be
called before initializing the rasterizer. This allows us to check what
rasterizer to use in OS::initialize together with the GL context
initialization.

This commit also adds a new project setting
"rendering/quality/driver/driver_fallback" which allows the creator of a
project to specify whether or not fallback to GLES2 is allowed. This
setting is ignored for the editor so the editor will always open even if
the project itself cannot run. This will hopefully reduce confusion for
users downloading projects from the internet.

We also no longer crash when GLES3 is not functioning on a platform.

This fixes #15324

5 files changed, 38 insertions, 28 deletions

diff --git a/drivers/dummy/rasterizer_dummy.h b/drivers/dummy/rasterizer_dummy.h
index e39ec915fc..0381d3f0c1 100644
--- a/drivers/dummy/rasterizer_dummy.h
+++ b/drivers/dummy/rasterizer_dummy.h
@@ -789,6 +789,10 @@ public:
 	void end_frame(bool p_swap_buffers) {}
 	void finalize() {}
 
+	static Error is_viable() {
+		return OK;
+	}
+
 	static Rasterizer *_create_current() {
 		return memnew(RasterizerDummy);
 	}
diff --git a/drivers/gles2/rasterizer_gles2.cpp b/drivers/gles2/rasterizer_gles2.cpp
index c926f2bcc4..76ee80aa07 100644
--- a/drivers/gles2/rasterizer_gles2.cpp
+++ b/drivers/gles2/rasterizer_gles2.cpp
@@ -136,26 +136,21 @@ RasterizerScene *RasterizerGLES2::get_scene() {
 	return scene;
 }
 
-void RasterizerGLES2::initialize() {
-
-	print_verbose("Using GLES2 video driver");
+Error RasterizerGLES2::is_viable() {
 
 #ifdef GLAD_ENABLED
 	if (!gladLoadGL()) {
 		ERR_PRINT("Error initializing GLAD");
+		return ERR_UNAVAILABLE;
 	}
 
 // GLVersion seems to be used for both GL and GL ES, so we need different version checks for them
 #ifdef OPENGL_ENABLED // OpenGL 2.1 Profile required
-	if (GLVersion.major < 2) {
-#else // OpenGL ES 3.0
+	if (GLVersion.major < 2 || (GLVersion.major == 2 && GLVersion.minor < 1)) {
+#else // OpenGL ES 2.0
 	if (GLVersion.major < 2) {
 #endif
-		ERR_PRINT("Your system's graphic drivers seem not to support OpenGL 2.1 / OpenGL ES 2.0, sorry :(\n"
-				  "Try a drivers update, buy a new GPU or try software rendering on Linux; Godot will now crash with a segmentation fault.");
-		OS::get_singleton()->alert("Your system's graphic drivers seem not to support OpenGL 2.1 / OpenGL ES 2.0, sorry :(\n"
-								   "Godot Engine will self-destruct as soon as you acknowledge this error message.",
-				"Fatal error: Insufficient OpenGL / GLES driver support");
+		return ERR_UNAVAILABLE;
 	}
 
 #ifdef GLES_OVER_GL
@@ -181,14 +176,21 @@ void RasterizerGLES2::initialize() {
 			glGetFramebufferAttachmentParameteriv = glGetFramebufferAttachmentParameterivEXT;
 			glGenerateMipmap = glGenerateMipmapEXT;
 		} else {
-			ERR_PRINT("Your system's graphic drivers seem not to support GL_ARB(EXT)_framebuffer_object OpenGL extension, sorry :(\n"
-					  "Try a drivers update, buy a new GPU or try software rendering on Linux; Godot will now crash with a segmentation fault.");
-			OS::get_singleton()->alert("Your system's graphic drivers seem not to support GL_ARB(EXT)_framebuffer_object OpenGL extension, sorry :(\n"
-									   "Godot Engine will self-destruct as soon as you acknowledge this error message.",
-					"Fatal error: Insufficient OpenGL / GLES driver support");
+			return ERR_UNAVAILABLE;
 		}
 	}
 #endif
+
+#endif // GLAD_ENABLED
+
+	return OK;
+}
+
+void RasterizerGLES2::initialize() {
+
+	print_verbose("Using GLES2 video driver");
+
+#ifdef GLAD_ENABLED
 	if (true || OS::get_singleton()->is_stdout_verbose()) {
 		if (GLAD_GL_ARB_debug_output) {
 			glEnable(_EXT_DEBUG_OUTPUT_SYNCHRONOUS_ARB);
@@ -198,7 +200,6 @@ void RasterizerGLES2::initialize() {
 			print_line("OpenGL debugging not supported!");
 		}
 	}
-
 #endif // GLAD_ENABLED
 
 	// For debugging
diff --git a/drivers/gles2/rasterizer_gles2.h b/drivers/gles2/rasterizer_gles2.h
index f727af39dd..98c73b776b 100644
--- a/drivers/gles2/rasterizer_gles2.h
+++ b/drivers/gles2/rasterizer_gles2.h
@@ -61,9 +61,10 @@ public:
 	virtual void end_frame(bool p_swap_buffers);
 	virtual void finalize();
 
+	static Error is_viable();
 	static void make_current();
-
 	static void register_config();
+
 	RasterizerGLES2();
 	~RasterizerGLES2();
 };
diff --git a/drivers/gles3/rasterizer_gles3.cpp b/drivers/gles3/rasterizer_gles3.cpp
index 0d42635194..e4824695d5 100644
--- a/drivers/gles3/rasterizer_gles3.cpp
+++ b/drivers/gles3/rasterizer_gles3.cpp
@@ -136,28 +136,32 @@ typedef void (*DEBUGPROCARB)(GLenum source,
 
 typedef void (*DebugMessageCallbackARB)(DEBUGPROCARB callback, const void *userParam);
 
-void RasterizerGLES3::initialize() {
-
-	print_verbose("Using GLES3 video driver");
+Error RasterizerGLES3::is_viable() {
 
 #ifdef GLAD_ENABLED
 	if (!gladLoadGL()) {
 		ERR_PRINT("Error initializing GLAD");
+		return ERR_UNAVAILABLE;
 	}
 
 // GLVersion seems to be used for both GL and GL ES, so we need different version checks for them
 #ifdef OPENGL_ENABLED // OpenGL 3.3 Core Profile required
-	if (GLVersion.major < 3 && GLVersion.minor < 3) {
+	if (GLVersion.major < 3 || (GLVersion.major == 3 && GLVersion.minor < 3)) {
 #else // OpenGL ES 3.0
 	if (GLVersion.major < 3) {
 #endif
-		ERR_PRINT("Your system's graphic drivers seem not to support OpenGL 3.3 / OpenGL ES 3.0, sorry :(\n"
-				  "Try a drivers update, buy a new GPU or try software rendering on Linux; Godot will now crash with a segmentation fault.");
-		OS::get_singleton()->alert("Your system's graphic drivers seem not to support OpenGL 3.3 / OpenGL ES 3.0, sorry :(\n"
-								   "Godot Engine will self-destruct as soon as you acknowledge this error message.",
-				"Fatal error: Insufficient OpenGL / GLES driver support");
+		return ERR_UNAVAILABLE;
 	}
 
+#endif // GLAD_ENABLED
+	return OK;
+}
+
+void RasterizerGLES3::initialize() {
+
+	print_verbose("Using GLES3 video driver");
+
+#ifdef GLAD_ENABLED
 	if (OS::get_singleton()->is_stdout_verbose()) {
 		if (GLAD_GL_ARB_debug_output) {
 			glEnable(_EXT_DEBUG_OUTPUT_SYNCHRONOUS_ARB);
@@ -167,7 +171,6 @@ void RasterizerGLES3::initialize() {
 			print_line("OpenGL debugging not supported!");
 		}
 	}
-
 #endif // GLAD_ENABLED
 
 	/* // For debugging
diff --git a/drivers/gles3/rasterizer_gles3.h b/drivers/gles3/rasterizer_gles3.h
index f4449ac0f9..0a264caf8f 100644
--- a/drivers/gles3/rasterizer_gles3.h
+++ b/drivers/gles3/rasterizer_gles3.h
@@ -62,9 +62,10 @@ public:
 	virtual void end_frame(bool p_swap_buffers);
 	virtual void finalize();
 
+	static Error is_viable();
 	static void make_current();
-
 	static void register_config();
+
 	RasterizerGLES3();
 	~RasterizerGLES3();
 };