Rename weston_output_region_from_global to weston_region_global_to_output,
and also no longer modify in place.
Trying to make it look a little nicer, as well as making it easier to use
from other places that don't want modify in place semantics.
This becomes a very thin wrapper around weston_matrix_transform_region.
Signed-off-by: Derek Foreman <derek.foreman@collabora.com>