Mouse OVER/OUT events are not sent when objects under the mouse change but the mouse is not moved

[nadako]

originally FOE-56180

Quite easy to reproduce, but non-trivial to fix:

• add a display
• attach mouse over/out events on it
• toggle its visibility without moving mouse
• results on Flash and OpenFL are different:
  • flash: will receive OVER/OUT events
  • openfl: will NOT receive them

This basically requires recalculating objects under the cursor on every display tree change, which is quite expensive. The hit-test bounds caching optimization (#154) could help with it if we manage to do it right.