Investigating the following gesture highlighted two slight implementation
errors with choosing which slots to report in which slot when multiple
contacts are present:

Action                 SGM  AGM (MTB slot:Contact)
1. Touch contact 0    (0:0)
2. Touch contact 1    (0:0, 1:1)
3. Lift  contact 0    (1:1)
4. Touch contacts 2,3 (0:2, 1:3)

In step 4, slot 1 was not being cleared first, which means the same
tracking ID was being used for reporting both the old contact 1 and the
new contact 3.  This could result in "drumroll", where the old contact 1
would appear to suddenly jump to new finger 3 position.

Similarly, if contacts 2 & 3 are not detected at the same sample, step 4
is split into two:

Action                SGM  AGM  (MTB slot:contact)
1. Touch contact 0   (0:0)
2. Touch contact 1   (0:0, 1:1)
3. Lift  contact 0   (1:1)
4. Touch contact 2   (0:2, 1:1)
5. Touch contact 3   (0:2, 1:3)

In this case, there was also a bug.  In step 4, when contact 1 moves from
SGM to AGM and contact 2 is first reported in SGM, slot 0 was actually
empty.  So slot 0 can be used to report the new SGM (contact 0),
immediately.  Since it was empty, contact 2 in slot 0 will get a new
tracking ID.
Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>