Further trackball fixes

@bsenftner  - I think I finally have a fix for scrolling.  Could you test?

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Previously the screen variable was not updated on scroll, which meant that top & left could be wrohng.  This updates it before access in the mouse events.

The relative position is then

event.pageX - screen.left, event.pageY - screen.top

I also added a snippet to calculate pageX/Y if needed