Button state -1 for no mouse buttons depressed

[bethge]

In the Pointer Events spec, section 5.1.1 it says, that if no mouse buttons are depressed, the button property of the event is supposed to be -1. If I am not mistaken, PEP currently only passes through the mouse event's button value (which is 0).

IE appears to have already extended the button state range to -1, see IE Dev Center Button Property.

I was wondering if you also intend to implement this behavior. (I believe @Steditor hinted at this in #168)

[Steditor]

This should be easy to implement with the changes of #168 (it could be a simple reverse mapping from buttons to button) - we should probably try to make some progress there...