Initial stab at tracking vertex alpha.

Not sure what efficient method to use on x86...
Switch to fullAlphaReg not hasAlphaReg.
Merge branch 'vertexalpha' of github.com:unknownbrackets/ppsspp into unknownbrackets-vertexalpha
Merge branch 'unknownbrackets-vertexalpha' into avoid-alpha-test

Conflicts:
	GPU/GLES/VertexDecoderX86.cpp