The Win 8 protocol specify the fact that each valid touch must be reported
within a frame until it is released.
We can therefore use the always_valid quirk and dismiss reports when we see
duplicate contacts ID.

We recognize Win8 certified devices from their vendor feature 0xff0000c5
where Microsoft put a signed blob in the report to check if the device
passed the certification.
Signed-off-by: NBenjamin Tissoires <benjamin.tissoires@gmail.com>
Reviewed-by: NHenrik Rydberg <rydberg@euromail.se>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>