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()
Get rid of depalFBOs when overwriting textures.

Just in case they are not always used as framebuffers.  We set to 0 later
anyway.
Track temporary-use FBOs all in FramebufferManager.

This way it's easier to manage their lifecycle.
Decimate temporary FBOs after not being used.
Doesn't look like these need to use depth/stencil.

May save a bit of VRAM/RAM.