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.
Merge pull request #6206 from unknownbrackets/framebuffer-clut

Track and decimate temporary-use FBOs in one place