Render to texture from viewpoint

[maluoi]

Render StereoKit's active scene to a render texture, given a matrix for camera view, and perspective! Think a DSLR viewfinder in VR.

Later on, will also want this for separate scenes, for doing things like rendering thumbnails.

[maluoi]

This is mostly working in 1b251ac. It's still upside-down on OpenGL systems though.

Also would be great to be able to render to a specific viewport on the texture, or to texture array layers! See LookingGlass's quilt concept:
https://docs.lookingglassfactory.com/KeyConcepts/quilts/

[maluoi]

One more feature left, which is partially implemented: rendering the scene with a material override