Update disabled IsReallyAClear() code.

Not sure if there's still something wrong about depth.
Add basic decimation to the depal clut cache.
Disable vertex arrays not in use.
Disable vertex arrays before depal as well.
Enable the glClear() optimization.

It seems to help and so far have not noticed any problems.
Oops, remove an old wrong comment.
Merge pull request #6202 from unknownbrackets/gpu-minor

Decimate in clut stuff, adjustments to IsReallyAClear()