swrast: fix readback regression since inversion fix

swrast: fix readback regression since inversion fix

This readback from the frontbuffer with swrast was broken, that bug
just made it more obviously broken, this fixes it by inverting the
sub image gets. Also fixes a few other piglits.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=72327
Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=72325
(for 9.2 the patches this depends on were asked to be backported separately
 in an email).
Cc: "9.2" "10.0" mesa-stable@lists.fedoraproject.org
Reviewed-by: Brian Paul <brianp@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>