Improve the tap detection mechanism.

Improve the tap detection mechanism.

Revision 1.29 of wstpad.c has removed the 'maxdist' checks
for multi-finger taps.  While this change makes tap detection
more reliable, and does not affect inputs intended for pointer
movement, it might interfere with short scroll gestures.
This version reorganizes the filtering code, and reintroduces
a weaker version of those checks for MT touchpads.