mesa: Track the TexImage being rendered to in the gl_renderbuffer.

mesa: Track the TexImage being rendered to in the gl_renderbuffer.

We keep having to pass the attachments around with our gl_renderbuffers
because that's the only way to find what the gl_renderbuffer actually
refers to.  This is a step toward removing that (though drivers still need
the Zoffset as well).

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>