If we have one global filter, one pointer may change the filter value and
affect another pointer.
Reproduceable effect:
blackbox and xterm, start dragging xterm then click anywhere with the other
pointer (attached to different masterd device!). The button release resets
the filter[Motion_Filter(button)] value, thus stopping dragging and no event
is sent to the client anymore.
Having the filters set per device gets around this.
f44d7dcb5f