mostly remove the svg onClick hack

- chrome no longer throws a security warning when trying to read pixel
  data on a canvas after drawing *any* svg; only svgs with a <foreignObject>
  tag in them
- Standard Safari is still an old enough WebKit that it has this problem,
  but WebKit-nightly (SVN r173347) does not.
- instead of the blanket check, just try it and catch errors

js/Sprite.js

@@ -234,12 +234,6 @@ Sprite.prototype.onClick = function(evt) {
     var mouseX = runtime.mousePos[0] + 240;
     var mouseY = 180 - runtime.mousePos[1];
 
-    if (this.mesh.src.indexOf('.svg') == -1) {
-        // HACK - if the image SRC doesn't indicate it's an SVG,
-        // then we'll try to detect if the point we clicked is transparent
-        // by rendering the sprite on a canvas.  With an SVG,
-        // we are forced not to do this for now by Chrome/Webkit SOP:
-        // http://code.google.com/p/chromium/issues/detail?id=68568
     var canv = document.createElement('canvas');
     canv.width = 480;
     canv.height = 360;
@@ -255,10 +249,15 @@ Sprite.prototype.onClick = function(evt) {
     ctx.translate(-rotationCenterX, -rotationCenterY);
     ctx.drawImage(this.mesh, 0, 0);
 
+    var alpha;
+    try {
         var idata = ctx.getImageData(mouseX, mouseY, 1, 1).data;
-        var alpha = idata[3];
-    } else {
-        var alpha = 1;
+        alpha = idata[3];
+    } catch (e) {
+        // as of 2014-09-06, WebKit/Safari still throws a security exception trying to read
+        // image data from a canvas after any svg has been draw to it. Version 7.0.6 (9537.78.2)
+        // WebKit-nightly (SVN-r173347) does not have this issue.
+        alpha = 1;
     }
 
     if (alpha > 0) {