Use COGL_FLAGS_* for the context's private feature flags

Previously the private feature flags were stored in an enum and we
already had 31 flags. Adding the 32nd flag would presumably make it
add -2³¹ as one of the values which might cause problems. To avoid
this we'll just use an fixed-size array of longs and use indices for
the enum values like we do for the public features.

A slight complication with this is in the CoglDriverDescription where
we were previously using a static intialised value to describe the set
of features that the driver supports. We can't easily do this with the
flags array so instead the features are stored in a fixed-size array
of indices.

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