This change finishes the big rename associated with the old
`compositing` crates. Long ago, these crates managed a compositor, like
you might find in a traditional web engine. These days, compositing is
done in WebRender so the name has stopped making much sense. Various
structs inside the crates have already been renamed and this is the
final big change necessary for the rename
Testing: This is just a rename so existing tests should cover it.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>