Bug 1497294 - P6. Fix NV12 playback on legacy D3D11 compositor. r=mattwoodrow

Depends on D8203

Differential Revision: https://phabricator.services.mozilla.com/D8246

--HG--
extra : moz-landing-system : lando

gfx/layers/d3d11/CompositorD3D11.cpp

@@ -867,6 +867,9 @@ CompositorD3D11::DrawGeometry(const Geometry& aGeometry,
 
       const float* yuvToRgb = gfxUtils::YuvToRgbMatrix4x3RowMajor(YUVColorSpace::BT601);
       memcpy(&mPSConstants.yuvColorMatrix, yuvToRgb, sizeof(mPSConstants.yuvColorMatrix));
+      // TOTO: need to handle color depth properly. this assumes data is always
+      // 8 or 16 bits.
+      mPSConstants.vCoefficient[0] = 1.0;
 
       SetSamplerForSamplingFilter(texturedEffect->mSamplingFilter);
     }