mouseScrolled in PJS program scrolls entire page

The canvas should trap mouse scroll events. Currently the only way to do this is

```
mouseScrolled = function() {
// jshint noarg:false
arguments.callee.caller.arguments[0].preventDefault();
};
```

which is a horrific abuse of `arguments` ~~and doesn't work in Firefox (I think the `callee.caller` function would need to return false or something)~~.

Listening to `mouseScrolled` would be useful for many things, but is basically useless because of this bug.

Start at the mouseScrolled event entry point and trace how scroll events from the canvas reach the page. Verify the event can be handled without the arguments.callee workaround, including Firefox behavior. Done means listening to mouseScrolled prevents the page from scrolling while preserving useful canvas scroll handling.

Written by the indexing model from the issue text.