meta: Refactor the BlitFramebuffer color CopyTexImage fallback.

meta: Refactor the BlitFramebuffer color CopyTexImage fallback.

There shouldn't be anything special about copying out a subset of the src
rb to a temp before texturing from it, so just do it when we're figuring
out our src texture binding.

This drops Anuj's change to copy an extra border of 1 pixel around the src
area.  I can't see how that change could be valid, and presumably if
there's some filtering problem at edges we just need to set the right
wrap mode.

v2: Don't fall back to swrast on non-2D/RECT/2D_MS textures when we can
    still CopyTexSubImage.  Fixes a segfault regression on i965 with
    gl-3.2-layered-rendering-blit.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com> (v1)
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org> (v1)
Tested-by: Anuj Phogat <anuj.phogat@gmail.com>