Minor GPU code cleanups
Add a conservative check that prevents alpha testing in a few cases.

This will become really powerful if we add some code to the vertex decoder
to check for non-full alpha in the vertices, and set gstate_c.vertexFullAlpha if none
is found (probably want to do the reverse, set it to true and clear if any non-255 alpha is found).

Alpha testing is a performance killer on many mobile GPUs so big efforts to
avoid it can be worth it.