Update the BTN_TOOL bits correctly during SYN_DROPPED handling

Update the BTN_TOOL bits correctly during SYN_DROPPED handling

Where at least one touch ends during SYN_DROPPED, we send out two event
frames: one with all applicable touch sequences ending (tracking id -1) and
the second one with the whole device state *and* the applicable touch
sequences starting (tracking id != -1).

This requires us to also update the BTN_TOOL_ bits correctly so that they are
correct after the first frame. For that we count the number of previously
known touches and send a 0 event for the matching BTN_TOOL_ bit, together with
a 1 event for the currently known touches.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>