Prevent touch screen translation from entering invalid state

Prevent touch screen translation from entering invalid state

* lisp/subr.el (touch-screen-events-received): New variable.
(read--potential-mouse-event): If a touch screen event's been
registered thus far, continue as though xterm-mouse-mode is
enabled.

* lisp/touch-screen.el (touch-screen-handle-touch): Set that
variable.  If t-s-c-t already exists but the new touch point was
assigned the same number by the system, replace the current tool
with it rather than installing it as the anciliary tool.