Sanitize coordinate handling

Now (0,0) is always the center of the trackpad.
Fixed button zones

 * It's the lowest touch, not the earliest, that determines the desired
   button number.
 * Invalid touches (especially in the bottom edge) also count.
 * Out-of-area touches do happen, don't map them to the last button.

The idea about the lowest touch belongs to PetteriAimonen.
Merge branch 'buttonzones-v2' of https://github.com/patrakov/xf86-input-mtrack into patrakov-buttonzones-v2

Conflicts:
	src/mtstate.c