Initial stab at tracking vertex alpha.

Not sure what efficient method to use on x86...
Switch to fullAlphaReg not hasAlphaReg.
Correctly keep track of "full alpha" in vertices (x86 jit only).
Merge branch 'vertexalpha' of github.com:unknownbrackets/ppsspp into unknownbrackets-vertexalpha
Merge branch 'unknownbrackets-vertexalpha' into avoid-alpha-test

Conflicts:
	GPU/GLES/VertexDecoderX86.cpp
Eliminate further alpha tests
ARM buildfix
More checks for alpha test elimination
Merge pull request #5710 from hrydgard/avoid-alpha-test

Avoid alpha test when vertexFullAlpha && textureFullAlpha