This partially reverts the change from a longer time ago to always build
DXVA2 and D3D11VA together.
To make it simpler, we change the following:
- building with ANGLE headers is now required to build D3D hwaccels
- if DXVA2 is enabled, D3D11VA is still forcibly built
- the CLI vo_opengl ANGLE backend is now under --egl-angle-win32
This is done to reduce the dependency mess slightly.