Make old findCanvasEventTarget work with OffscreenCanvases

Author: juj

src/library_html5.js

@@ -338,10 +338,10 @@ var LibraryJSEvents = {
   _findCanvasEventTarget: function(target) {
     if (typeof target === 'number') target = UTF8ToString(target);
     if (!target || target === '#canvas') {
-      if (typeof GL !== 'undefined' && GL.offscreenCanvases['canvas']) return GL.offscreenCanvases['canvas']; // TODO: Remove this line, target '#canvas' should refer only to Module['canvas'], not to GL.offscreenCanvases['canvas'] - but need stricter tests to be able to remove this line.
+      if (typeof GL !== 'undefined' && GL.offscreenCanvases['canvas']) return GL.offscreenCanvases['canvas']['offscreenCanvas']; // TODO: Remove this line, target '#canvas' should refer only to Module['canvas'], not to GL.offscreenCanvases['canvas'] - but need stricter tests to be able to remove this line.
       return Module['canvas'];
     }
-    if (typeof GL !== 'undefined' && GL.offscreenCanvases[target]) return GL.offscreenCanvases[target];
+    if (typeof GL !== 'undefined' && GL.offscreenCanvases[target]) return GL.offscreenCanvases[target]['offscreenCanvas'];
     return __findEventTarget(target);
   },
 #endif