Point light shadow mapping now utilizes cube-to-2D projection, avoiding the need for a samplerCube in the shader.

Added checks for definition of POINT_LIGHT_SHADOWS in shaders for including point light shadow mapping code.

* Shorter (less typo-prone)
* More intuitive (also "aligns" with gl.POINTS)
* Makes a nice API (Mesh, Line, Points).

Remove unneeded function calls, rearrage for fused multiply–add, remove
excessive shader whitespace