Generalize mesh mapping to allow for pixel space view mapping

[glopesdev]

When using non-linear display surfaces such as spherical projection domes, each pixel in the projector output will be independently mapped to a viewing direction in the world. Currently the mesh mapping solution allows correcting this distortion using a grid of vertices that warp the final image texture.

However, given that we have a cubemap handy, the precision of this mapping can be improved by mapping each pixel fragment independently to a direction in the cubemap, essentially doing the distortion of the view directions in the fragment shader instead of the pixel shader.