A Synaptics image sensor tracks 5 fingers, but can only report 2.

The algorithm for choosing which 2 fingers to report and in which packet:
  Touchpad maintains 5 slots, numbered 0 to 4
  Initially all slots are empty
  As new fingers are detected, assign them to the lowest available slots
  The touchpad always reports:
    SGM: lowest numbered non-empty slot
    AGM: highest numbered non-empty slot, if there is one

In addition, these touchpads have a special AGM packet type which reports
the number of fingers currently being tracked, and which finger is in
each of the two slots.  Unfortunately, these "TYPE=2" packets are only used
when more than 3 fingers are being tracked.  When less than 4 fingers
are present, the 'w' value must be used to track how many fingers are
present, and knowing which fingers are being reported is much more
difficult, if not impossible.
Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Acked-by: Chase Douglas <chase.douglas@canonical.com>
Acked-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>