Support alpha / color test masks on desktop/gles3.

Won't work on GLES 2, though...
Pre-mask alphatest value as a small optimization.

Might cause small behavior difference on gles2 (without bitwise ops.)
Fix a crash in the GE debugger on matrix tab.
Merge pull request #6341 from unknownbrackets/gpu-minor

Support alpha and color test masks on desktop/gles3