when dragging on the canvas, use x,y pointer coordinates to decide if motion has occured.

Using _last_pointer_frame breaks when dragging to the left of the canvas, because we clamp
the value of the frame to >= 0. Motion would step once the pointer crossed the left edge
of the canvas because the frame value would always be zero.

This is not a problem when using the pointer x,y values which end up appropriately negative
under all conditions.