wayland: Add a convenience function to update a region from SHM buffer

Adds cogl_wayland_texture_set_region_from_shm_buffer which is a
convenience wrapper around cogl_texture_set_region but it uses the
correct format to copy the data from a Wayland SHM buffer. This will
typically be used by compositors to update the texture for a surface
when an SHM buffer is attached. The ordering of the arguments is based
on cogl_texture_set_region_from_bitmap.

Based on a patch by Jasper St. Pierre.

Reviewed-by: Robert Bragg <robert@linux.intel.com>