Fix double proximity error
As found in xournalpp there is a specific event sequence that can cause two tools to be in proximity at the same time. The sequence is:
- pen in prox
- pen forced out of proximity by timeout
- pen actually prox out
- eraser in prox but without any axis updates, just the
BTN_TOOL_RUBBER
- eraser moves
This fell through a wrong if condition and caused the pen to be triggered back into proximity.
Fixes #418 (closed)