Drop pixman submodule and support for the "internal" pixman build.
pixman should be reasonably well established meanwhile so we don't
need the fallback submodule any more.  While being at it also drop
some #ifdefs for pixman versions older than what we require in
configure anyway.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 20170905140116.28181-2-kraxel@redhat.com