swrast: fix incorrectly positioned putImage() in swrast driver

swrast: fix incorrectly positioned putImage() in swrast driver

Some front buffer rendering was in the wrong position.  This included
scissored clears, glDrawPixels and glCopyPixels.  The problem was the
y coordinate passed to putImage() didn't match the y coordinate passed
to getImage().

We fix this by setting xrb->map_y to the inverted coordinate in
swrast_map_renderbuffer() which is used later by the putImage() call.
Also pass xrb->map_y to getImage() to be symmetric.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97426
Cc: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Eric Anholt <eric@anholt.net>