compositor-drm: expose global shadow flag for pixman

Allow global control of the pixman shadow buffers. The compositor can
choose whether all output use or do not use a shadow buffer with the
pixman renderer.

The option is added to the end of struct weston_drm_backend_config to
avoid bumping WESTON_DRM_BACKEND_CONFIG_VERSION.

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>