Input: appletouch - fix jumps when additional fingers are detected

Addresses issues related to when a second finger enters or leaves the
field, causing the cursor to jump or the page to scroll unexpectedly; now,
we discard any movement change that happens at the exact moment we detect a
change in the number of fingers touching the trackpad. This doesn't
completely resolve the issue but does greatly mitigate it.

Signed-off-by: Clinton Sprain <clintonsprain@gmail.com>
Reviewed-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>