Fixed mouseover for when top canvases don't enable mouseover, but bottom do.

Go ahead with testMouseOver in more cases than before: instead of just
skipping it (as a "redundant listener") whenever a previous stage was
defined, we now skip it only if at least one of the previous stages has
_mosueOverIntervalID defined.
In the cases where no previous stages have mouseOverIntervalID defined
(found = false), we, in the second loop I added, compute eventTarget by
checking the previous stages. This way a stage enabled with mouseover
which is buried under other stages can find the right canvas to set the
cursor style of (it's the canvas of a previous stage which matches the
event target).

Not sure if this will be helpful or not since you recently said you were already working on mouse events to work better with layered canvases. These changes seemed to work for my project.