Hardware-accelerated Skia RenderTargetBitmap

[kekekeks]

For now it was disabled because of threading issues and memory corruption: https://github.com/AvaloniaUI/Avalonia/pull/2615

It can be easily reproduced on Linux with GLX and requires some less trivial steps on Windows with EGL/ANGLE. Haven't checked on OSX, but I suspect that the firework there would be even more spectacular.

I had several attempts to fix it in rtb-gl-fixes branch, but it seems that we need to have two GrContext instances with appropriate locking model.

[kekekeks]

Basically we need something like this:

with proper locks to avoid concurrent access to: 1) GrContext 2) OpenGL 3) SkSurface 4) OpenGL texture

Also, we need to make sure that GrContext/OpenGL context are properly switched when drawing context is created and destroyed.

[kekekeks]

Note: some requirements that any other proposed solution has to meet

• We need to be able to render to a texture on UI thread and consume the same texture on the render thread
• We also need two separate OpenGL contexts, so rendering to RTB on UI thread won't prevent the - render thread from drawing some completely unrelated window
• We also need a way to switch between "render thread" (Deferred) and "ui thread" (Immediate) contexts while being on the UI thread to process the window painting request from the OS ASAP