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

1 files changed, 48 insertions, 14 deletions

diff --git a/platform/javascript/os_javascript.cpp b/platform/javascript/os_javascript.cpp
index 5a8a05d4df..80699b0d32 100644
--- a/platform/javascript/os_javascript.cpp
+++ b/platform/javascript/os_javascript.cpp
@@ -652,23 +652,57 @@ Error OS_JavaScript::initialize(const VideoMode &p_desired, int p_video_driver,
 	attributes.alpha = false;
 	attributes.antialias = false;
 	ERR_FAIL_INDEX_V(p_video_driver, VIDEO_DRIVER_MAX, ERR_INVALID_PARAMETER);
-	switch (p_video_driver) {
-		case VIDEO_DRIVER_GLES3:
-			attributes.majorVersion = 2;
-			RasterizerGLES3::register_config();
-			RasterizerGLES3::make_current();
-			break;
-		case VIDEO_DRIVER_GLES2:
-			attributes.majorVersion = 1;
-			RasterizerGLES2::register_config();
-			RasterizerGLES2::make_current();
-			break;
+
+	bool gles3 = true;
+	if (p_video_driver == VIDEO_DRIVER_GLES2) {
+		gles3 = false;
+	}
+
+	bool gl_initialization_error = false;
+
+	while (true) {
+		if (gles3) {
+			if (RasterizerGLES3::is_viable() == OK) {
+				attributes.majorVersion = 2;
+				RasterizerGLES3::register_config();
+				RasterizerGLES3::make_current();
+				break;
+			} else {
+				if (GLOBAL_GET("rendering/quality/driver/driver_fallback") == "Best") {
+					p_video_driver = VIDEO_DRIVER_GLES2;
+					gles3 = false;
+					continue;
+				} else {
+					gl_initialization_error = true;
+					break;
+				}
+			}
+		} else {
+			if (RasterizerGLES2::is_viable() == OK) {
+				attributes.majorVersion = 1;
+				RasterizerGLES2::register_config();
+				RasterizerGLES2::make_current();
+				break;
+			} else {
+				gl_initialization_error = true;
+				break;
+			}
+		}
 	}
 
-	video_driver_index = p_video_driver;
 	EMSCRIPTEN_WEBGL_CONTEXT_HANDLE ctx = emscripten_webgl_create_context(NULL, &attributes);
-	ERR_EXPLAIN("WebGL " + itos(attributes.majorVersion) + ".0 not available");
-	ERR_FAIL_COND_V(emscripten_webgl_make_context_current(ctx) != EMSCRIPTEN_RESULT_SUCCESS, ERR_UNAVAILABLE);
+	if (emscripten_webgl_make_context_current(ctx) != EMSCRIPTEN_RESULT_SUCCESS) {
+		gl_initialization_error = true;
+	}
+
+	if (gl_initialization_error) {
+		OS::get_singleton()->alert("Your browser does not support any of the supported WebGL versions.\n"
+								   "Please update your browser version.",
+				"Unable to initialize Video driver");
+		return ERR_UNAVAILABLE;
+	}
+
+	video_driver_index = p_video_driver;
 
 	video_mode = p_desired;
 	// Can't fulfil fullscreen request during start-up due to browser security.