vo_gpu: d3d11: initial implementation

This is a new RA/vo_gpu backend that uses Direct3D 11. The GLSL
generated by vo_gpu is cross-compiled to HLSL with SPIRV-Cross.

What works:

- All of mpv's internal shaders should work, including compute shaders.

- Some external shaders have been tested and work, including RAVU and
  adaptive-sharpen.

- Non-dumb mode works, even on very old hardware. Most features work at
  feature level 9_3 and all features work at feature level 10_0. Some
  features also work at feature level 9_1 and 9_2, but without high-bit-
  depth FBOs, it's not very useful. (Hardware this old is probably not
  fast enough for advanced features anyway.)

  Note: This is more compatible than ANGLE, which requires 9_3 to work
  at all (GLES 2.0,) and 10_1 for non-dumb-mode (GLES 3.0.)

- Hardware decoding with D3D11VA, including decoding of 10-bit formats
  without truncation to 8-bit.

What doesn't work / can be improved:

- PBO upload and direct rendering does not work yet. Direct rendering
  requires persistent-mapped PBOs because the decoder needs to be able
  to read data from images that have already been decoded and uploaded.
  Unfortunately, it seems like persistent-mapped PBOs are fundamentally
  incompatible with D3D11, which requires all resources to use driver-
  managed memory and requires memory to be unmapped (and hence pointers
  to be invalidated) when a resource is used in a draw or copy
  operation.

  However it might be possible to use D3D11's limited multithreading
  capabilities to emulate some features of PBOs, like asynchronous
  texture uploading.

- The blit() and clear() operations don't have equivalents in the D3D11
  API that handle all cases, so in most cases, they have to be emulated
  with a shader. This is currently done inside ra_d3d11, but ideally it
  would be done in generic code, so it can take advantage of mpv's
  shader generation utilities.

- SPIRV-Cross is used through a NIH C-compatible wrapper library, since
  it does not expose a C interface itself.

  The library is available here: https://github.com/rossy/crossc

- The D3D11 context could be made to support more modern DXGI features
  in future. For example, it should be possible to add support for
  high-bit-depth and HDR output with DXGI 1.5/1.6.

wscript_build.py

@@ -401,8 +401,12 @@ def build(ctx):
         ( "video/out/cocoa_common.m",            "cocoa" ),
         ( "video/out/dither.c" ),
         ( "video/out/filter_kernels.c" ),
+        ( "video/out/d3d11/context.c",           "d3d11" ),
+        ( "video/out/d3d11/hwdec_d3d11va.c",     "d3d11 && d3d-hwaccel" ),
+        ( "video/out/d3d11/ra_d3d11.c",          "d3d11" ),
         ( "video/out/opengl/angle_dynamic.c",    "egl-angle" ),
         ( "video/out/gpu/context.c" ),
+        ( "video/out/gpu/d3d11_helpers.c",       "d3d11 || egl-angle-win32" ),
         ( "video/out/gpu/hwdec.c" ),
         ( "video/out/gpu/lcms.c" ),
         ( "video/out/gpu/osd.c" ),
@@ -432,7 +436,6 @@ def build(ctx):
         ( "video/out/opengl/context_glx.c",      "gl-x11" ),
         ( "video/out/opengl/context_x11egl.c",   "egl-x11" ),
         ( "video/out/opengl/cuda_dynamic.c",     "cuda-hwaccel" ),
-        ( "video/out/opengl/d3d11_helpers.c",    "egl-angle-win32" ),
         ( "video/out/opengl/egl_helpers.c",      "egl-helpers" ),
         ( "video/out/opengl/hwdec_cuda.c",       "cuda-hwaccel" ),
         ( "video/out/opengl/hwdec_d3d11egl.c",   "d3d-hwaccel" ),