The Mir backend was checking for button mask changes to generate the appropriate GDK event. When Mir generates a touch event it has no button mask. In this case we'll just generate a primary button event.
If we get a Mir event we don't understand generate a warning instead of an assert.
This is disabled by default. Enable with --enable-mir-backend. This backend is a combined work of Robert Ancell and Ryan Lortie.
