Add WGL context creation and OpenGL support

[Zoxc]

This adds WGL context creation and desktop OpenGL support to the gles backend. It's used instead of EGL on Windows. Only WGL requests desktop OpenGL contexts. The behavior of EGL is left unchanged.

To enable construction of an OpenGL context without a surface, a memory device is used. Each surface gets their own OpenGL context which share object with the memory device OpenGL context. Rendering to surfaces happens indirectly via renderbuffers as framebuffers cannot be shared across OpenGL contexts.

Currently a lot of gles backend features are not enabled on desktop OpenGL.

I've tested it with an HD 5870 using the examples and vello.

bunnymark is plain blue. skybox crashes in the driver. mipmap is too white, Vsync locks to 30 fps instead of 60 fps on a 60 Hz display. These may be issues with the driver or gles backend rather than this PR however.

[Zoxc]

I added some more feature detection and driver workarounds. That makes pass tests for vger-rs.

[teoxoy]

Is it possible to test this in CI?

[cwfitzgerald]

@teoxoy not yet. Unless we manage to set up gl-on-12 over warp, this will need to wait until we have a physical box with a real gpu running our windows tests.

[cwfitzgerald]

I am very happy to be wrong! GLon12 isn't terribly hard to set up, though definitely shouldn't be part of this PR. Will file a follow up with more information.

[Zoxc]

I've changed the PR to use a dummy window instead of a memory device, as that makes OpenGL context creation work on Mesa and my Sandy Bridge laptop.

[cwfitzgerald]

Pushed a commit that enables renderdoc with wgl.

Outside of presentation being all black on nvidia, this is looking good.

[cwfitzgerald]

This is super amazing stuff. Tested successfully on all of my devices!

Everything is fairly minor

[teoxoy]

Could you also add an "OpenGL" row to the "Supported Platforms" table in the readme (and remove the GLES3 Angle entry for windows)?

[cwfitzgerald]

Stunning work! Thank you for getting this all through!