Handle FF Ubuntu deviation for mouseup

[bethge]

Tackles #242.

Not sure if you'd rather want to wait for a response from FF. I believe this PR should not break the current behaviour (incl. chorded button presses), while fixing the situation for Ubuntu FF.

• button says which button triggers the mouseup
• buttons says which buttons (in summation) are used

Cases:

• If buttons includes the lifted button's value:
  • buttons === BUTTON_TO_BUTTONS[button]:
    • The triggering button is the last one in use and afterwards no buttons are pressed anymore
  • buttons !== BUTTON_TO_BUTTONS[button]:
    • The triggering button is not the last active button
• If buttons does not include the lifted button's value:
  • buttons === BUTTON_TO_BUTTONS[button]:
    • Not possible: The triggering button's value is by definition not part of buttons. Every possible value of buttons uniquely identifies one specific state (bitmap). Thus no other combination of buttons can cause the buttons value.
  • buttons !== BUTTON_TO_BUTTONS[button]:
    • Not relevant

I hope these cases are sound.