tilt: minor cleanup / handle bad tablet input
tilt: minor cleanup / handle bad tablet input

and don't call all those math functions when there is no tilt

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