[VR] Fixed Foveated Mask Rendering

[RobertBeckebans]

Right now the openvr3 branch is mainly pixel shader limited. NVRHI allows Variable Rate Shading (VRS) and it is very easy to implement. We only need a precalculated mask to reduce the resolution for the pixels at the corners.

References:
https://developers.meta.com/horizon/blog/tech-note-mask-based-foveated-rendering-with-unreal-engine-4-/

https://mariosbikos.com/vive-unreal-foveated-rendering/

[RobertBeckebans]

Code to write the mask can be found here:
https://github.com/fholger/openvr_foveated/blob/542768df39864386deebf260e73cf5158851fb9c/src/vrs/VariableRateShading.cpp#L50