Add pointer input to layer surfaces
This glues things together in rootston and extends the layer shell example to visually demonstrate pointer events.
Test plan:
- Run rootston
- Run layer shell example with various configurations
- Test that pointer appears in correct place and clicking changes the color of the surface
- Test that surface gets a client side cursor
- Bring a shell surface to the party and make sure nothing breaks
- Test z-ordering works correctly with several layer surfaces