evdev: Don't transform device->abs.x/y in place

We don't always get both an X and an Y event in a SYN report, so we end
up transforming the coordinate we don't get twice.  For example, if we
only receive an ABS_X event, we transform the already transformed
device->abs.y again in transform_absolute() when applying the calibration.

src/evdev.c

@@ -236,23 +236,21 @@ is_motion_event(struct input_event *e)
 }
 
 static void
-transform_absolute(struct evdev_device *device)
+transform_absolute(struct evdev_device *device, int32_t *x, int32_t *y)
 {
-	int32_t x, y;
-
-	if (!device->abs.apply_calibration)
+       if (!device->abs.apply_calibration) {
+               *x = device->abs.x;
+               *y = device->abs.y;
                return;
-
-	x = device->abs.x * device->abs.calibration[0] +
+       } else {
+               *x = device->abs.x * device->abs.calibration[0] +
                        device->abs.y * device->abs.calibration[1] +
                        device->abs.calibration[2];
 
-	y = device->abs.x * device->abs.calibration[3] +
+               *y = device->abs.x * device->abs.calibration[3] +
                        device->abs.y * device->abs.calibration[4] +
                        device->abs.calibration[5];
-
-	device->abs.x = x;
-	device->abs.y = y;
+       }
 }
 
 static void
@@ -260,6 +258,7 @@ evdev_flush_motion(struct evdev_device *device, uint32_t time)
 {
        struct weston_seat *master = device->seat;
        wl_fixed_t x, y;
+       int32_t cx, cy;
        int slot;
 
        if (!(device->pending_events & EVDEV_SYN))
@@ -299,10 +298,9 @@ evdev_flush_motion(struct evdev_device *device, uint32_t time)
                device->pending_events &= ~EVDEV_ABSOLUTE_MT_UP;
        }
        if (device->pending_events & EVDEV_ABSOLUTE_MOTION) {
-		transform_absolute(device);
+               transform_absolute(device, &cx, &cy);
                weston_output_transform_coordinate(device->output,
-						   device->abs.x,
-						   device->abs.y, &x, &y);
+                                                  cx, cy, &x, &y);
 
                if (device->caps & EVDEV_TOUCH) {
                        if (master->num_tp == 0)