pixman: update internal copy to pixman-0.28.2

Some w64 fixes by Stefan Weil found their way into 0.28.2,
so update the internal copy to that version to improve
windows support.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>