Only process each mouse up & down event once at either canvas or window

Internet explorer will only trigger mouse events at the window level. If
the handler is bound to both window and canvas however, the event will
be processed twice in every other browser.

A solution to this is to track manually (without affecting the bubble or
other features on the event) that an event has been processed, and don't
process it a second time.
Merge pull request #199 from peitschie/click-dup

Only process each mouse up & down event once at either canvas or window