After a scene reload a mouse cursor updates is performed via a
InputEventMouseMotion, that is exposed to the user.
The state of Input is however not adjusted to this InputEventMouseMotion
which can lead to inconsistencies.
This PR makes sure, that it is not exposed to the user.
It utilizes the method of Viewport::_process_picking for marking
events that are not sent to the user, so that this function doesn't
need to be changed.