meta: Split _swrast_BlitFramebuffer out of the meta blit path.

meta: Split _swrast_BlitFramebuffer out of the meta blit path.

Separating the software fallbacks from the rest of the meta path (which
is usually hardware accelerated) gives callers better control over their
blitting options.

For example, i965 might want to try meta blit, hardware blits, then
swrast as a last resort.  Splitting it makes that possible.

This updates all callers to maintain the existing behavior (even in the
few cases where it isn't desirable behavior - later patches can change
that).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>
Cc: "10.2" <mesa-stable@lists.freedesktop.org>