[WPE][WebGL][Canvas][Compositing] Fix partially rendered WebGL canvas…

[neodesys]

[WPE][WebGL][Canvas][Compositing] Fix partially rendered WebGL canvas composition
https://bugs.webkit.org/show_bug.cgi?id=250314

Reviewed by NOBODY (OOPS!).

When rendering heavy 3D scenes in a WebGL canvas using the WPE EGL
renderer, the compositor may use the element render target texture
BEFORE all the WebGL commands are correctly executed by the GPU.

Then, on most complex frames, the canvas appears totally blank or
partially rendered. This patch ensures that all GL commands from
the WebGL canvas context are finished before using the render target
texture in the compositor.

• Source/WebCore/platform/graphics/gbm/GraphicsContextGLGBM.cpp:
  (WebCore::GraphicsContextGLGBM::prepareTexture):

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< bug title >
https://bugs.webkit.org/show_bug.cgi?id=#####

Reviewed by NOBODY (OOPS!).

Explanation of why this fixes the bug (OOPS!).

* path/changed.ext:
(function):
(class.function):

9d7fc9b

Misc	iOS, tvOS & watchOS	macOS	Linux	Windows
✅ 🧪 style	✅ 🛠 ios	✅ 🛠 mac	✅ 🛠 wpe	❌ 🛠 🧪 win
✅ 🧪 bindings	✅ 🛠 ios-sim	✅ 🛠 mac-AS-debug	✅ 🛠 gtk	✅ 🛠 wincairo
✅ 🧪 webkitperl	✅ 🧪 ios-wk2	✅ 🧪 api-mac	✅ 🧪 gtk-wk2
	✅ 🧪 api-ios	✅ 🧪 mac-wk1	✅ 🧪 api-gtk
	✅ 🛠 tv	✅ 🧪 mac-wk2
	✅ 🛠 tv-sim	✅ 🧪 mac-AS-debug-wk2
	✅ 🛠 watch	✅ 🧪 mac-wk2-stress
	✅ 🛠 watch-sim

[webkit-early-warning-system]

EWS run on previous version of this PR (hash 87184bd)

Misc	iOS, tvOS & watchOS	macOS	Linux	Windows
✅ 🧪 style	✅ 🛠 ios	✅ 🛠 mac	✅ 🛠 wpe	❌ 🛠 🧪 win
✅ 🧪 bindings	✅ 🛠 ios-sim	✅ 🛠 mac-AS-debug	✅ 🛠 gtk	✅ 🛠 wincairo
✅ 🧪 webkitperl	✅ 🧪 ios-wk2	✅ 🧪 api-mac	✅ 🧪 gtk-wk2
	✅ 🧪 api-ios	❌ 🧪 mac-wk1	✅ 🧪 api-gtk
	✅ 🛠 tv	✅ 🧪 mac-wk2
	✅ 🛠 tv-sim	✅ 🧪 mac-AS-debug-wk2
	✅ 🛠 watch	✅ 🧪 mac-wk2-stress
	✅ 🛠 watch-sim

[neodesys]

I've added a comment in Bugzilla https://bugs.webkit.org/show_bug.cgi?id=250314#c5 about all possible solutions available to fix this issue. All possible solutions I can foresee for now have their advantages and drawbacks, none is perfect. It depends on the concrete use case.

This current implementation follows the solution 1a. It guaranties frame completeness in all conditions, but at the price of lower performances as it blocks the rendering thread (which may also be the webview main thread) until all WebGL commands have been executed for the corresponding canvas in the view.

We should discuss about which solution would be the best "all purposes" solution, knowing that if only glFinish is available (no fences), only current solution (1a) can be implemented.

[webkit-early-warning-system]

EWS run on current version of this PR (hash 9d7fc9b)

Misc	iOS, tvOS & watchOS	macOS	Linux	Windows
✅ 🧪 style	✅ 🛠 ios	✅ 🛠 mac	✅ 🛠 wpe	❌ 🛠 🧪 win
✅ 🧪 bindings	✅ 🛠 ios-sim	✅ 🛠 mac-AS-debug	✅ 🛠 gtk	✅ 🛠 wincairo
✅ 🧪 webkitperl	✅ 🧪 ios-wk2	✅ 🧪 api-mac	✅ 🧪 gtk-wk2
	✅ 🧪 api-ios	✅ 🧪 mac-wk1	✅ 🧪 api-gtk
	✅ 🛠 tv	✅ 🧪 mac-wk2
	✅ 🛠 tv-sim	✅ 🧪 mac-AS-debug-wk2
	✅ 🛠 watch	✅ 🧪 mac-wk2-stress
	✅ 🛠 watch-sim