Encode viewspace Z normal to preserve negative Z (fixes all light shifting errors, and shiny environment map errors due to lack of negative Z)

missed a few spots: cleanup: get rid of the no-op 'mex' function in the shaders which was being used as a dummy replacement for 'mix'

cleanup: get rid of the no-op 'mex' function in the shaders which was being used as a dummy replacement for 'mix'

a bunch of trivial clean-ups and commentary.

Coalesced patchset.

Pathfinding visualization WIP -- add many controls for x-ray render, add lighting, and better combat z-fighting and noise from wireframes and overlays