JS puzzles: use the PointerEvent API if available.

If the browser knows what 'PointerEvent' means, then we switch our
'onmousefoo' event handlers to the 'onpointerfoo' events, for both the
puzzle canvas and the resize handle.

The immediate effect of this is that we get to use the
setPointerCapture method on the puzzle canvas, in preference to the
deprecated Firefox-only setCapture.

A pointer event also contains extra fields compared to a mouse event:
as well as telling you which pointing device the event comes from, it
can also provide extra information, such as pressure, or the angle of
a stylus if the hardware can detect it. I don't have any immediate
ideas about what those could be used for, but it can't hurt to have
them available just in case we think of something in future.

emccpre.js

@@ -285,6 +285,11 @@ function dialog_cleanup() {
 }
 
 function set_capture(element, event) {
+    if (element.setPointerCapture !== undefined &&
+        event.pointerId !== undefined) {
+        element.setPointerCapture(event.pointerId);
+        return;
+    }
     if (element.setCapture !== undefined) {
         element.setCapture(true);
         return;
@@ -317,7 +322,7 @@ function initPuzzle() {
         return toret;
     };
 
-    onscreen_canvas.onmousedown = function(event) {
+    var canvas_mousedown_handler = function(event) {
         if (event.button >= 3)
             return;
 
@@ -334,9 +339,10 @@ function initPuzzle() {
 
         set_capture(onscreen_canvas, event);
     };
+
     var mousemove = Module.cwrap('mousemove', 'boolean',
                                  ['number', 'number', 'number']);
-    onscreen_canvas.onmousemove = function(event) {
+    var canvas_mousemove_handler = function(event) {
         var down = buttons_down();
         if (down) {
             var xy = canvas_mouse_coords(event, onscreen_canvas);
@@ -346,7 +352,7 @@ function initPuzzle() {
     };
     var mouseup = Module.cwrap('mouseup', 'boolean',
                                ['number', 'number', 'number']);
-    onscreen_canvas.onmouseup = function(event) {
+    var canvas_mouseup_handler = function(event) {
         if (event.button >= 3)
             return;
 
@@ -358,6 +364,16 @@ function initPuzzle() {
         }
     };
 
+    if (PointerEvent !== undefined) {
+        onscreen_canvas.onpointerdown = canvas_mousedown_handler;
+        onscreen_canvas.onpointermove = canvas_mousemove_handler;
+        onscreen_canvas.onpointerup = canvas_mouseup_handler;
+    } else {
+        onscreen_canvas.onmousedown = canvas_mousedown_handler;
+        onscreen_canvas.onmousemove = canvas_mousemove_handler;
+        onscreen_canvas.onmouseup = canvas_mouseup_handler;
+    }
+
     // Set up keyboard handlers. We call event.preventDefault()
     // in the keydown handler if it looks like we might have
     // done something with the key.  This means that users
@@ -665,7 +681,7 @@ function initPuzzle() {
         var restore_puzzle_size = Module.cwrap('restore_puzzle_size',
                                                'void', []);
         resize_handle.oncontextmenu = function(event) { return false; }
-        resize_handle.onmousedown = function(event) {
+        var resize_mousedown_handler = function(event) {
             if (event.button == 0) {
                 var xy = element_coords(onscreen_canvas);
                 resize_xbase = xy.x + onscreen_canvas.offsetWidth / 2;
@@ -680,7 +696,7 @@ function initPuzzle() {
             set_capture(resize_handle, event);
             event.preventDefault();
         };
-        window.addEventListener("mousemove", function(event) {
+        var resize_mousemove_handler = function(event) {
             if (resize_xbase !== null && resize_ybase !== null) {
                 var dpr = window.devicePixelRatio || 1;
                 resize_puzzle(
@@ -693,8 +709,8 @@ function initPuzzle() {
                     window.getSelection().removeAllRanges();
                 else
                     document.selection.empty();        }
-        });
+        };
-        window.addEventListener("mouseup", function(event) {
+        var resize_mouseup_handler = function(event) {
             if (resize_xbase !== null && resize_ybase !== null) {
                 resize_xbase = null;
                 resize_ybase = null;
@@ -714,7 +730,17 @@ function initPuzzle() {
                 }, 20);
                 event.preventDefault();
             }
-        });
+        };
+
+        if (PointerEvent !== undefined) {
+            resize_handle.onpointerdown = resize_mousedown_handler;
+            window.addEventListener("pointermove", resize_mousemove_handler);
+            window.addEventListener("pointerup", resize_mouseup_handler);
+        } else {
+            resize_handle.onmousedown = resize_mousedown_handler;
+            window.addEventListener("mousemove", resize_mousemove_handler);
+            window.addEventListener("mouseup", resize_mouseup_handler);
+        }
     }
 
     var rescale_puzzle = Module.cwrap('rescale_puzzle', 'void', []);