Pass around texture pointers, not addresses.

This makes it possible to override it easier, rather than needing it in
PSP RAM.
Consider the texture changed when framebuf changes.

Needed to properly consider render to self (should rebind if the
framebuffer is later used with a separate target.)
Duplicate framebuffers textured onto themselves.

This fixes #2917.

I verified that, at least for my card, blitting is much faster than
glReadPixels (by quite a margin.)
Clear the duplicate FBO on shutdown.