Register ``pinch to zoom'' touch screen gestures

* doc/emacs/input.texi (Touchscreens): Address pinch gestures.

* doc/lispref/commands.texi (Touchscreen Events): Address touch
screen pinch events and the process by which they are produced.

* java/org/gnu/emacs/EmacsWindow.java (figureChange)
<ACTION_POINTER_DOWN>: Supply pointer index to getX and getY,
correcting a mistake where the first touch point's coordinate
was saved here in lieu of the pointer that was pressed's.

* lisp/touch-screen.el (touch-screen-current-tool): Revise doc
string.
(touch-screen-aux-tool): New variable.
(touch-screen-scroll-point-to-y, touch-screen-pinch): New
functions.
(global-map): Bind [touchscreen-pinch] to touch-screen-pinch.
(touch-screen-handle-point-update): Revise doc string; set new
tenth field of t-s-c-t to POINT relative to its window, without
regard to whether an event has been sent.
(touch-screen-distance, touch-screen-centrum): New functions.
(touch-screen-handle-aux-point-update): New function; generate
and send touchscreen-pinch if need be.
(touch-screen-handle-point-up): If an ancillary tool exists,
transfer the information there into touch-screen-current-tool
and clear t-s-a-t.
(touch-screen-handle-touch): Call t-s-a-p-u as is proper; set
t-s-a-t if a touchscreen-down event arrives and t-s-c-t is set.

* src/androidterm.c (handle_one_android_event): Properly save
the event's X and Y when a new touch point is registered.

java/org/gnu/emacs/EmacsWindow.java

@@ -918,8 +918,8 @@ public final class EmacsWindow extends EmacsHandleObject
 	   it in the map.  */
 	pointerIndex = event.getActionIndex ();
 	pointerID = event.getPointerId (pointerIndex);
-	coordinate = new Coordinate ((int) event.getX (0),
-				     (int) event.getY (0),
+	coordinate = new Coordinate ((int) event.getX (pointerIndex),
+				     (int) event.getY (pointerIndex),
 				     buttonForEvent (event),
 				     pointerID);
 	pointerMap.put (pointerID, coordinate);