dix: leave last.valuators alone on slave switch

Terms:
dev->last.valuator[] is the last value given to us by the driver
dev->valuator.axisVal[] is the last value sent to the client
dev->last.scroll[] is the abs value of the scroll axis as given by the driver,
        used for button emulation calculation (and the remainder)

This function updates the device's last.valuator state based on the current
master axis state. This way, relative motion continues fluidly when switching
between devices. Before mouse 2 comes into effect, it's valuator state is
updated to wherever the pointer currently is so the relative event applies on
top of that.

This can only work for x/y axes, all other axes aren't guaranteed to have the
same meaning and/or may not be present:
- xtest device: no valuator 2
- mouse: valuator 2 is horizontal scroll axis
- tablet: valuator 2 is pressure

Scaling the current value from the pressure range into the range for
horizontal scrolling makes no sense. And it causes scroll jumps:

- scroll down, last.valuator == axisVal == 20
- xdotool click 1, the XTest device doesn't have that valuator
- scroll up
  - updateSlaveDeviceCoords reset last.valuator to 0 (axisVal == 20)
  - DeviceClassesChangedEvent includes value 20 for the axis
  - event is processed, last.value changes from 0 to -1
  - axisVal is updated to -1, causing a jump of -21

The same applies when we switch from tablet to mouse wheel if the pressure
value is 0 on proximity out (basically guaranteed). So let's drop this code
altogether and only leave the scaling for the relative x/y motion.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
(cherry picked from commit d7b1753d)
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