Refactor FlRenderer to platform-independent implementation

[huanghongxun]

This pull request is part of #23985.

This pull request refactors FlRenderer to a platform-independent implementation.

1. Switch to embedder compositor.
2. Introduced FlGLArea to represent a Flutter content layer from compositor.

Code changes:

1. Removes egl_utils.h/cc since I'm using libepoxy instead, which is also used by GTK. mock_egl.cc is refactored into mock_epoxy.cc for mocking libepoxy.
2. Drop out wayland-client and x11 requirement. Now we use gtk_cairo to draw OpenGL textures.
3. FlBackingStoreProvider for framebuffer and texture creation. Just like FlutterFrameBufferProvider in macOS implementation.
4. FlEngine now enables compositor capability of embedder.
5. FlGLArea for framebuffer and texture drawing. every FlGLArea has a GdkWindow, because we may embed some widgets provided by plugins, which have own GdkWindows.
6. FlRenderer is now responsible for compositing. Creates backing store by FlBackingStoreProvider, and present layers by putting FlGLAreas with given textures into FlView.
7. FlRendererGL for OpenGL renderer, creates GdkGLContext as OpenGL context.
8. FlView now is GtkContainer to support multiple layers of FlGLArea, maintaining a cached gl_area_list, and children_list. Every frame FlRenderer will reset children of FlView, but if every time we unparent and parent these child widgets, GtkInspector will crash. So calculates diff of children_list is necessary.
9. Pointer events now is captured by a GtkEventBox on top of FlView, instead of directly capturing events in FlView, because FlGLAreas have their own GdkWindows, which can capture pointer events, we must cover a GdkWindow for event capturing on the top.

This pr is part of platform view implementation on Linux, related issue: flutter/flutter#41724.

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[huanghongxun]

@robert-ancell This is the first part of pr to implement platform views on Linux. I have tried to minimize modifications.. But implementing compositor requires all of these changes.

[huanghongxun]

Linux Web Engine
recipe infra failure: Recipe timed out

All tests are passed now.

[wmww]

Is "collect" used elsewhere in Flutter to mean release? If not I think the meaning of this name is unintuitive.

[huanghongxun]

This function implements

engine/shell/platform/embedder/embedder.h

Line 1050 in 0bf71f6

FlutterBackingStoreCollectCallback collect_backing_store_callback;

[wmww]

Cool, in that case sounds good

[wmww]

I notice you unbind the texture at the end of the function, but don't unbind the framebuffer. It doesn't seem to be documented that this leaves the framebuffer bound. Is this intentional?

[huanghongxun]

Binding framebuffer 0 does not mean 'unbinding' framebuffer, but binding to the system default framebuffer. Flutter/Skia will always bind a specific framebuffer (provided by FlBackingStoreProvider) before rendering. So we have no need to bind to framebuffer 0 here. Restoring framebuffer binding requires a call to glGetIntegerv(GL_FRAMEBUFFER_BINDING, &old_fbo);.

macOS implementation has the same behavior, and because framebuffer 0 is not the default framebuffer.

engine/shell/platform/darwin/macos/framework/Source/FlutterFrameBufferProvider.mm

Lines 31 to 37 in 0bf71f6

	glBindFramebuffer(GL_FRAMEBUFFER, _frameBufferId);
	glBindTexture(GL_TEXTURE_RECTANGLE_ARB, _backingTexture);
	glTexParameterf(GL_TEXTURE_RECTANGLE_ARB, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
	glTexParameterf(GL_TEXTURE_RECTANGLE_ARB, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
	glTexParameteri(GL_TEXTURE_RECTANGLE_ARB, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
	glTexParameteri(GL_TEXTURE_RECTANGLE_ARB, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
	glBindTexture(GL_TEXTURE_RECTANGLE_ARB, 0);

[wmww]

Does geometry get set anywhere? I see it's used, but I don't see it being set.

[huanghongxun]

Now geometry is used in FlGLArea for resolving drawing texture repeatedly.

[wmww]

Looks sus to use error twice without checking it. probably best to check error after each call.

In practice I don't think this is unsafe or a memory leak, but if the 1st call results in an error the 2nd call will fail a non-fatal assertion inside GTK (which prints an error message to console). Might leak an error if assertions are turned off.

[huanghongxun]

Added error check right after line 24.

[wmww]

Tested and Wayland works on Sway and Mir. One oddity is that the GL view seems to be in tiled mode, so when you're resizing the app to be bigger (which is still al little janky) instead of being black, white or whatever was there before, the top and/or right edge is a copy of the other edge side of the window.

[huanghongxun]

Tested and Wayland works on Sway and Mir. One oddity is that the GL view seems to be in tiled mode, so when you're resizing the app to be bigger (which is still al little janky) instead of being black, white or whatever was there before, the top and/or right edge is a copy of the other edge side of the window.

Now resizing the app to be bigger, area not covered will be white.

[wmww]

LGTM

[robert-ancell]

LGTM, thanks @huanghongxun!

[huanghongxun]

Maybe we need your approval? @stuartmorgan @cbracken

[stuartmorgan-g]

@robert-ancell is a committer, so his review is sufficient unless there's a reason someone else needs to review it.