Do not store a second copy of old touchstrip values in oldTilt[XY]

The touchstrip values are already stored in oldStrip[XY], and appear to
also be stored in oldTilt[XY] as a quirk of history. The value of oldTilt[XY]
itself is only read in wcmSendNonPadEvents (which is not called for the pad
device for obvious reasons ;)). At this point in time, there is absolutely
no reason to store a second copy of the strip values in oldTilt[XY].

Signed-off-by: Jason Gerecke <killertofu@gmail.com>
Reviewed-by: Ping Cheng <pingc@wacom.com>
Remove unnecesary device variables: current[XY]

These variables are currently used only within wcmUpdateOldState
and can be passed in as arguments rather than held as variables
in the device struct.

Signed-off-by: Jason Gerecke <killertofu@gmail.com>
Reviewed-by: Ping Cheng <pingc@wacom.com>
Replace multitude of "old state" vars with a _WacomDeviceState

Reducing clutter.

Signed-off-by: Jason Gerecke <killertofu@gmail.com>
Reviewed-by: Ping Cheng <pingc@wacom.com>
Update values of oldState by simple assignment

When updating the value of oldState, it is sufficient to simply
use an assignment statement for the entire struct rather than
explicitly setting each field. A static const "out of prox"
state is created to define the default out-of-prox values.

Signed-off-by: Jason Gerecke <killertofu@gmail.com>
Reviewed-by: Ping Cheng <pingc@wacom.com>