Fix error about setCapture not existing.

element.setCapture only seems to exist in Firefox. On most other
browsers, our attempt to call it must have been generating a whinge in
the console log all along. But Ben's commit bb16b5a70ddf77d turned
that into a prominent alert box, triggered on every mouse click in the
puzzle canvas.

Worked around by wrapping both calls to setCapture in a local
subroutine which checks if it's there before calling it.

Also, setCapture turns out to be deprecated in any case, according to
https://developer.mozilla.org/en-US/docs/Web/API/Element/setCapture .
It looks as if the non-deprecated version is element.setPointerCapture:
https://developer.mozilla.org/en-US/docs/Web/API/Element/setPointerCapture
But it also looks as if that needs the 'pointerId' field that's only
found in 'onpointerdown' events and not 'onmousedown' ones. So
including that as an alternative will be a bigger job.

emccpre.js

@@ -284,6 +284,13 @@ function dialog_cleanup() {
     onscreen_canvas.focus();
 }
 
+function set_capture(element, event) {
+    if (element.setCapture !== undefined) {
+        element.setCapture(true);
+        return;
+    }
+}
+
 // Init function called early in main().
 function initPuzzle() {
     // Construct the off-screen canvas used for double buffering.
@@ -325,7 +332,7 @@ function initPuzzle() {
             event.preventDefault();
         button_phys2log[event.button] = logbutton;
 
-        onscreen_canvas.setCapture(true);
+        set_capture(onscreen_canvas, event);
     };
     var mousemove = Module.cwrap('mousemove', 'boolean',
                                  ['number', 'number', 'number']);
@@ -670,7 +677,7 @@ function initPuzzle() {
             } else {
                 restore_pending = true;
             }
-            resize_handle.setCapture(true);
+            set_capture(resize_handle, event);
             event.preventDefault();
         };
         window.addEventListener("mousemove", function(event) {