dix: reset last.scroll when resetting the valuator (#45611)

last.scroll remained on the last-submitted scrolling value but last.valuator
was changed whenever the slave device changed. The first scrolling delta
after a switch was then calculated as (last.scroll - new abs value), causing
erroneous scrolling events.

Test case:
- synaptics with a scrolling method enabled, other device with 3+ axes (e.g.
  wacom)
- scroll on touchpad
- use other device
- scroll on touchpad

The second scroll caused erroneous button press/release events.

X.Org Bug 45611 <http://bugs.freedesktop.org/show_bug.cgi?id=45611>

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Chase Douglas <chase.douglas@canonical.com>

dix/getevents.c

@@ -360,6 +360,7 @@ updateSlaveDeviceCoords(DeviceIntPtr master, DeviceIntPtr pDev)
             if (i >= lastSlave->valuator->numAxes)
             {
                 pDev->last.valuators[i] = 0;
+                valuator_mask_set_double(pDev->last.scroll, i, 0);
             }
             else
             {
@@ -367,6 +368,7 @@ updateSlaveDeviceCoords(DeviceIntPtr master, DeviceIntPtr pDev)
                 val = rescaleValuatorAxis(val, lastSlave->valuator->axes + i,
                                           pDev->valuator->axes + i, 0, 0);
                 pDev->last.valuators[i] = val;
+                valuator_mask_set_double(pDev->last.scroll, i, val);
             }
         }
     }