Currently mouse hovering doesn't update the state, when collision objects
or the camera move.
This PR fixes this problem by taking the mouse position from the viewport
and not from a nonexistent previous event.
Since previous events could potentially be a long time ago, their
modifier-key state might be outdated. This PR fetches the current
status of modifier-keys from `Input`.
These changes allow the removal of some class-variables and making
additional simplifications.
543fdc1490