Hololens feature request: Prime depth buffer /w spatial surfaces (room)

[mlfarrell]

I tried doing this on the OpenGL side, and the rendering performance of my living room's meshes are too poor due to Angle's OpenGL's draw call overhead & GLSL->HLSL shader conversion overhead. Ideally this should be done inside of ANGLE with a glEnable target perhaps. This allows the implementation of the spatial surface rendering to be highly efficient (like in this SDK example: https://github.com/Microsoft/Windows-universal-samples/tree/master/Samples/HolographicSpatialMapping)