Disable generation of mouse-1 events after ancillary tool presses

Disable generation of mouse-1 events after ancillary tool presses

* lisp/touch-screen.el (touch-screen-handle-scroll): Correct
typo in comment.
(touch-screen-handle-point-update): Attempt to detect scroll
gestures and the like also when what is ancillary-tool.
(touch-screen-handle-point-up): Don't needlessly compute
relative xy when transforming aux tool into current tool.
(touch-screen-handle-touch): Set what field to ancillary-tool
rather than clear it when initializing the ancillary tool,
indicating to t-s-h-p-u that it must not generate mouse up
events.