Whenever we cross from N slots to at least one fake finger, reset the motion
history and skip the next event too. Especially on serial Synaptics touchpads,
the first touch update after a two-slot → TRIPLETAP is garbage, as is the one
from TRIPLETAP → two slots.

Example sequence reproduce on a T440s:

E: 4.488757 0003 003a 0084      # EV_ABS / ABS_MT_PRESSURE      84
E: 4.488757 0003 002f 0001      # EV_ABS / ABS_MT_SLOT          1
E: 4.488757 0003 0039 0433      # EV_ABS / ABS_MT_TRACKING_ID   433
E: 4.488757 0003 0035 2500      # EV_ABS / ABS_MT_POSITION_X    2500
E: 4.488757 0003 0036 3064      # EV_ABS / ABS_MT_POSITION_Y    3064
E: 4.488757 0003 003a 0060      # EV_ABS / ABS_MT_PRESSURE      60
E: 4.488757 0003 0018 0084      # EV_ABS / ABS_PRESSURE         84
E: 4.488757 0001 0145 0000      # EV_KEY / BTN_TOOL_FINGER      0
E: 4.488757 0001 014e 0001      # EV_KEY / BTN_TOOL_TRIPLETAP   1
E: 4.488757 0000 0000 0000      # ------------ SYN_REPORT (0) ----------
E: 4.508506 0003 002f 0000      # EV_ABS / ABS_MT_SLOT          0
E: 4.508506 0003 0036 2982      # EV_ABS / ABS_MT_POSITION_Y    2982
E: 4.508506 0003 003a 0086      # EV_ABS / ABS_MT_PRESSURE      86
E: 4.508506 0003 002f 0001      # EV_ABS / ABS_MT_SLOT          1
E: 4.508506 0003 0035 3464      # EV_ABS / ABS_MT_POSITION_X    3464
E: 4.508506 0003 0036 2716      # EV_ABS / ABS_MT_POSITION_Y    2716
E: 4.508506 0003 0001 2982      # EV_ABS / ABS_Y                2982
E: 4.508506 0003 0018 0086      # EV_ABS / ABS_PRESSURE         86
E: 4.508506 0000 0000 0000      # ------------ SYN_REPORT (0) ----------

subsequent events then hover around the 3464 mark, but that initial jump is
enough to cause a massive cursor jump.

https://bugs.freedesktop.org/show_bug.cgi?id=91352



Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Hallelujah-expressed-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>