brushlib: Call mypaint_tiled_surface_update_tile() after read-only _get_tile()
brushlib: Call mypaint_tiled_surface_update_tile() after read-only _get_tile()

All get_tile/update_tile are now done in pairs, so implementations
can sanely keep state between them.

b3sigma/libmypaint,achadwick/libmypaint,achadwick/libmypaint,achadwick/libmypaint,achadwick/libmypaint,b3sigma/libmypaint,b3sigma/libmypaint