staging/content-type: Content type hint support

This protocol lets clients advertise which kind of content they expect
to be displayed on a given surface, so that the compositor can make more
informed decisions about its behavior and output configuration.

Signed-off-by: Emmanuel Gil Peyrot <linkmauve@linkmauve.fr>
Signed-off-by: Xaver Hugl <xaver.hugl@gmail.com>