Platform view implementation on Linux

[huanghongxun]

This pull request aims at supporting platform views on Linux.

To support platform views:

1. We replaced original FlRenderer implementation (creating EGLSurface and EGLContext by X11 and Wayland) with a new and platform-independent implementation (creating OpenGL context by GdkGLContext, and draw textures to FlGLArea)
2. Now Flutter Linux window may have multiple FlGLAreas as backing store layer and GtkWidgets from plugins as platform view layer. They are stacked in FlView which extends GtkContainer now.
3. We deal with gestures like the way in iOS. First we listen to gtk pointer events, send it to Flutter framework side, accept gesture if gesture detector test hit positively on the platform view, and finally embedder will send all stored pointer events to platform view(GtkWidget) by gtk_widget_event.
4. No longer requires EGL, wayland-client, x11 now. libepoxy is added for GL operations, such as creating Framebuffers and textures, and procs for Skia.

This pr closes flutter/flutter#41724.

Pre-launch Checklist

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• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
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• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt. See [testing the engine] for instructions on
  writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
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• All existing and new tests are passing.
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[robert-ancell]

Thanks @huanghongxun! Can you split this PR into smaller PRs, perhaps starting with the FlGLArea change? It's a lot of changes to review at once.

[robert-ancell]

@wmww - I think we'll need some of your help reviewing this.

[wmww]

Not a full review, but I tested and the default demo app works on Wayland (both Sway and Mir). It removes my dirty hacks for Wayland and seems to make resizing a little less flickery. Only oddity is

Status 8cd5

is dumped to the terminal repeatedly.

[huanghongxun]

For FlGLArea changes, please see #24011

[huanghongxun]

For platform view implementation, please see #24169.

[chinmaygarde]

I am a bit confused about whether this or #24169 is the right issue to track the implementation of platform views on Linux. Based on the review activity, it seems like the other one is the way to go? If so, can we close this one please?