main: add setting for DRM/pixman shadow framebuffer

Allows to control the Pixman-renderer shadow framebuffer usage from
weston.ini. It defaults to enabled, and whether it is a good idea to
disable or not depends on the platform and the workload.

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>