The previous event processing had subtle issues with touches stopping during
SYN_DROPPED. All of the device state was processed in the same frame, but if
any touch changed tracking ID during SYN_DROPPED, an inserted SYN_REPORT
resulted in a weird split of events:
- the first frame had all key/sw/abs updates including those slots that
  changed tracking ID, but not the ones that were fully terminated.
- the second frame had only the slots states for newly started touches **and**
  the slot state for touches terminated during SYN_DROPPED but not restarted.

In other words, where three fingers were on the touchpad and slot 0 was lifted
and put down again and slot 1 was lifted but *not* put down again, our frames
contained:
- frame 1: terminate slot 0, BTN_TOOL_TRIPLETAP 0, BTN_TOOL_DOUBLETAP 1
- frame 2: start slot 0, terminate slot 1

Where there was no touch changing tracking ID, only one frame was generated.
The BTN_TOOL updates were buggy, they may not match the number of fingers down
as seen on a frame-by-frame basis. This triggered libinput bug
libinput/libinput#422



This patch changes the above example to
- frame 1: terminate slot 0, terminate slot 1
- frame 2: start slot 0, BTN_TOOL_TRIPLETAP 0, BTN_TOOL_DOUBLETAP 1

Notably, the first frame no longer contains the BTN_TOOL bits. This patch is
one of two, the BTN_TOOL sync bits are part of a follow-up patch.

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