Fix multiwindow support in GLES3 for X11, Windows, and MacOS.

Instead of updating all viewports, then blitting all viewports
to the backbuffer, then swapping all buffers, we run through
all viewports and render, blit, and swap backbuffer before
going to the next viewport.

3 files changed, 14 insertions, 22 deletions

diff --git a/platform/linuxbsd/gl_manager_x11.cpp b/platform/linuxbsd/gl_manager_x11.cpp
index 04c1df71fb..838be2c042 100644
--- a/platform/linuxbsd/gl_manager_x11.cpp
+++ b/platform/linuxbsd/gl_manager_x11.cpp
@@ -256,7 +256,11 @@ void GLManager_X11::release_current() {
 	if (!_current_window) {
 		return;
 	}
-	glXMakeCurrent(_x_windisp.x11_display, None, nullptr);
+
+	if (!glXMakeCurrent(_x_windisp.x11_display, None, nullptr)) {
+		ERR_PRINT("glXMakeCurrent failed");
+	}
+	_current_window = nullptr;
 }
 
 void GLManager_X11::window_make_current(DisplayServer::WindowID p_window_id) {
@@ -276,7 +280,9 @@ void GLManager_X11::window_make_current(DisplayServer::WindowID p_window_id) {
 
 	const GLDisplay &disp = get_display(win.gldisplay_id);
 
-	glXMakeCurrent(disp.x11_display, win.x11_window, disp.context->glx_context);
+	if (!glXMakeCurrent(disp.x11_display, win.x11_window, disp.context->glx_context)) {
+		ERR_PRINT("glXMakeCurrent failed");
+	}
 
 	_internal_set_current_window(&win);
 }
@@ -290,13 +296,12 @@ void GLManager_X11::make_current() {
 		return;
 	}
 	const GLDisplay &disp = get_current_display();
-	glXMakeCurrent(_x_windisp.x11_display, _x_windisp.x11_window, disp.context->glx_context);
+	if (!glXMakeCurrent(_x_windisp.x11_display, _x_windisp.x11_window, disp.context->glx_context)) {
+		ERR_PRINT("glXMakeCurrent failed");
+	}
 }
 
 void GLManager_X11::swap_buffers() {
-	// NO NEED TO CALL SWAP BUFFERS for each window...
-	// see https://www.khronos.org/registry/OpenGL-Refpages/gl2.1/xhtml/glXSwapBuffers.xml
-
 	if (!_current_window) {
 		return;
 	}
@@ -315,13 +320,6 @@ void GLManager_X11::swap_buffers() {
 		}
 	}
 
-	//	print_line("\tswap_buffers");
-
-	// only for debugging without drawing anything
-	//	glClearColor(Math::randf(), 0, 1, 1);
-	//glClear(GL_COLOR_BUFFER_BIT);
-
-	//const GLDisplay &disp = get_current_display();
 	glXSwapBuffers(_x_windisp.x11_display, _x_windisp.x11_window);
 }
 
diff --git a/platform/macos/gl_manager_macos_legacy.mm b/platform/macos/gl_manager_macos_legacy.mm
index e6bb7aaa85..dec4821b86 100644
--- a/platform/macos/gl_manager_macos_legacy.mm
+++ b/platform/macos/gl_manager_macos_legacy.mm
@@ -167,9 +167,8 @@ void GLManager_MacOS::make_current() {
 }
 
 void GLManager_MacOS::swap_buffers() {
-	for (const KeyValue<DisplayServer::WindowID, GLWindow> &E : windows) {
-		[E.value.context flushBuffer];
-	}
+	GLWindow &win = windows[current_window];
+	[win.context flushBuffer];
 }
 
 void GLManager_MacOS::window_update(DisplayServer::WindowID p_window_id) {
diff --git a/platform/windows/gl_manager_windows.cpp b/platform/windows/gl_manager_windows.cpp
index d509ff8c51..7689751f1b 100644
--- a/platform/windows/gl_manager_windows.cpp
+++ b/platform/windows/gl_manager_windows.cpp
@@ -289,12 +289,7 @@ void GLManager_Windows::make_current() {
 }
 
 void GLManager_Windows::swap_buffers() {
-	// on other platforms, OpenGL swaps buffers for all windows (on all displays, really?)
-	// Windows swaps buffers on a per-window basis
-	// REVISIT: this could be structurally bad, should we have "dirty" flags then?
-	for (KeyValue<DisplayServer::WindowID, GLWindow> &entry : _windows) {
-		SwapBuffers(entry.value.hDC);
-	}
+	SwapBuffers(_current_window->hDC);
 }
 
 Error GLManager_Windows::initialize() {