compositor: add capability CAPTURE_YFLIP

compositor: add capability CAPTURE_YFLIP

Both GL and pixman renderer (pixman probably only because GL did?)
return the screen capture image as y-flipped, therefore Weston y-flips
it again. However, the future rpi-renderer can produce only right-way-up
(non-flipped) screen captures, and does not need an y-flip.

Add a capability flag for y-flip, which the rpi-renderer will not set,
to get screen captures the right way up.

The wcap recording code needs yet another temporary buffer for the
non-flipped case, since the WCAP format is flipped, and the code
normally overwrites the input image as it compresses it. This becomes
difficult, if the compressor is supposed to flip while processing.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>