Prevent 'warning X4000: use of potentially uninitialized variable' console message cause by early return statment in glsl shadow code

WebGLRenderer: Reverted isArrayCamera handling again. Seems like fewer program changes is better than (many) more uniform calls.

WebGLRenderer: Reverted to previous approach for rendering ArrayCamera. The current renderer architecture is not ready.

Per https://github.com/mrdoob/three.js/issues/11367#issuecomment-308303716
Equirectangular U sampling update to intuitively equiangular-per-raster sampling.

A quick visual test shows no change from the old version, which worries me a bit, since I'm not getting the trig-on-paper to give identical results.  If the math DOES work out provably identically, the original version is certainly more efficient.