vo_gpu: hwdec_dxva2dxgi: initial implementation

This enables DXVA2 hardware decoding with ra_d3d11. It should be useful
for Windows 7, where D3D11VA is not available. Images are transfered
from D3D9 to D3D11 using D3D9Ex surface sharing[1].

Following Microsoft's recommendations, it uses a queue of shared
surfaces, similar to Microsoft's ISurfaceQueue. This will hopefully
prevent surface sharing from impacting parallelism and allow multiple
D3D11 frames to be in-flight at once.

[1]: https://msdn.microsoft.com/en-us/library/windows/desktop/ee913554.aspx

DOCS/man/options.rst

@@ -691,8 +691,9 @@ Video
     :videotoolbox: requires ``--vo=gpu`` (OS X 10.8 and up),
                    or ``--vo=opengl-cb`` (iOS 9.0 and up)
     :videotoolbox-copy: copies video back into system RAM (OS X 10.8 or iOS 9.0 and up)
-    :dxva2:     requires ``--vo=gpu`` with ``--gpu-context=angle`` or
-                ``--gpu-context=dxinterop`` (Windows only)
+    :dxva2:     requires ``--vo=gpu`` with ``--gpu-context=d3d11``,
+                ``--gpu-context=angle`` or ``--gpu-context=dxinterop``
+                (Windows only)
     :dxva2-copy: copies video back to system RAM (Windows only)
     :d3d11va:   requires ``--vo=gpu`` with ``--gpu-context=d3d11`` or
                 ``--gpu-context=angle`` (Windows 8+ only)