* MouseRegion documentation claimed that onEnter and onExit
  would track entry and exit regardless of whether the pointer was
  down or up
* It did such, but when grabbing the value of `event.down` from
  the passed event, the value was always `false`
* PointerEnterEvent and PointerExitEvent were overriding the value
  passed from PointerEvent in constructors, even if the value was true
  e.g. in invocations of .fromMouseEvent((PointerMoveEvent...))
* This change now passes the value along to PointerEnter/ExitEvents
  while providing it a default of false, and updates documentation

Fixes #40637