Use angle GPU context

[Kagami]

Right now ANGLE backend is disabled in custom mpv build because it requires d3dcompiler_47.dll, libEGL.dll and libGLESv2.dll which weight 2.5M compressed. Also it takes more time to start mpv with that backend, at least in VM.

But @wiiaboo mentioned that win backend doesn't always work good (current default in boram), so it's probably worth to enable it again. In order to avoid library duplications, it should be possible to add top-level electron's directory to the DLL PATH or something like that (Electron uses exactly same DLLs for its OpenGL rendering).

[Kagami]

Also required for #9

[Kagami]

Also consider:

--gpu-context=d3d11
--gpu-context=angle
--gpu-context=winvk
--gpu-context=dxinterop

Do they provide any real benifits compared to --gpu-context=win except hwdec?

dxva2 requires  --vo=gpu  with  --gpu-context=d3d11, --gpu-context=angle or --gpu-context=dxinterop (Windows only)

d3d11va requires --vo=gpu with --gpu-context=d3d11 or --gpu-context=angle (Windows 8+ only)