down logic

For the MT protocol, we need to properly keep track of each down touch.
This change simplifies the logic, and should make things easier when
support for the Magic Trackpad is added.

Signed-off-by: Chase Douglas <chase.douglas@canonical.com>
Acked-by: Michael Poole <mdpoole@troilus.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
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