Add quirk to control velocity averaging, disable it by default

libinput applies averaging to the velocity of most pointer devices. Averaging the velocity makes the motion look smooth and may be of benefit to bad input devices. For good devices, however, it comes at the unfortunate price of decreased accuaracy.

This change turns velocity averaging off by default (sets ntrackers to 2 instead of 16) and allows for it to be turned back on via a quirk, for bad devices which require it.