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

wscript_build.py

@@ -384,6 +384,7 @@ def build(ctx):
         ( "video/out/filter_kernels.c" ),
         ( "video/out/d3d11/context.c",           "d3d11" ),
         ( "video/out/d3d11/hwdec_d3d11va.c",     "d3d11 && d3d-hwaccel" ),
+        ( "video/out/d3d11/hwdec_dxva2dxgi.c",   "d3d11 && d3d9-hwaccel" ),
         ( "video/out/d3d11/ra_d3d11.c",          "d3d11" ),
         ( "video/out/opengl/angle_dynamic.c",    "egl-angle" ),
         ( "video/out/gpu/context.c" ),