pixman: avoid unnecessary y-flip for screen capture

Commit 4fc5dd00 ("compositor: add capability CAPTURE_YFLIP")
introduced a capability flag which indicates whether y-flipping is
necessary. As already indicated in that commit message, it seems
that pixman flipps the y-axis only due to historic reasons.

Drop y-flipping and use the WESTON_CAP_CAPTURE_YFLIP flag to
indicate that y-flipping is not necessary. This simplifies code
and improves screen share performance (on my test by about 3% down
to 18% CPU load on the sharing instance of Weston).

Signed-off-by: Stefan Agner <stefan@agner.ch>
2 jobs for screen-share-improvements in 1 minute and 36 seconds (queued for 1 minute and 43 seconds)
Status Job ID Name Coverage
  Container Prep
passed #463928


passed #463929