xwayland: reduce over-damage

If an X11 app draws a little here, some there, and a tiny bit in the
opposite corner, using RegionExtents for the damage to be sent to the
Wayland compositor will cause massive over-damaging.

However, we cannot blindly send an arbitrary number of damage
rectangles, because there is a risk of overflowing the Wayland
connection. If that happens, it triggers an abort in libwayland-client.

Try to be more accurate with the damage by sending up to 256 rectangles
per window, and fall back to extents otherwise. The number is completely
arbitrary.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>