pixman,drm: do not composite previous damage

Pixman-renderer uses a single internal shadow buffer. It is enough to
composite the current damage into shadow, but the copy to hw buffer
needs to include the previous damage because of double-buffering in
DRM-backend.

This patch lets pixman-renderer do exactly that without compositing also
the previous damage on DRM-renderer.

Arguably weston_output should not have field previous_damage to begin
with, because it implies double-buffering, which e.g. EGL does not
guarantee. It would be better for each backend explicitly always provide
any extra damage that should be copied to hw.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Signed-off-by: Fabien Lahoudere <fabien.lahoudere@collabora.com>
Reviewed-by: Ian Ray <ian.ray@ge.com>