Calculate the speed of the touch and compare it against a fixed speed limit.
If a touch exceeds the speed when a second touch is set down, that second
touch is marked as a thumb and ignored (unless it's right next to the other
finger, then it's likely a 2fg scroll).

The speed calculation is simple but has to lag behind by one sample - we reset
the motion history whenever a new finger is set down (to avoid pointer jumps)
so we need to know if the finger was moving fast *before* this happens. Plus,
with the pointer jumps we're more likely to get false positives if we
calculate the speed on actual finger down.

This is the simplest version for now, the speed varies greatly between
movements and should probably be averaged across the last 3-or-so samples.

https://bugs.freedesktop.org/show_bug.cgi?id=99703



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