backends/libinput: don't apply pointer acceleration to touchpad gestures

It's unexpected that increasing pointer "speed" would also change the distance
required to activate gestures

autotests/libinput/mock_libinput.cpp

@@ -486,6 +486,16 @@ double libinput_event_gesture_get_dy(struct libinput_event_gesture *event)
     return 0.0;
 }
 
+double libinput_event_gesture_get_dx_unaccelerated(struct libinput_event_gesture *event)
+{
+    return libinput_event_gesture_get_dx(event);
+}
+
+double libinput_event_gesture_get_dy_unaccelerated(struct libinput_event_gesture *event)
+{
+    return libinput_event_gesture_get_dy(event);
+}
+
 double libinput_event_gesture_get_scale(struct libinput_event_gesture *event)
 {
     switch (event->type) {

src/backends/libinput/events.cpp

@@ -268,8 +268,8 @@ int GestureEvent::fingerCount() const
 
 QPointF GestureEvent::delta() const
 {
-    return QPointF(libinput_event_gesture_get_dx(m_gestureEvent),
-                   libinput_event_gesture_get_dy(m_gestureEvent));
+    return QPointF(libinput_event_gesture_get_dx_unaccelerated(m_gestureEvent),
+                   libinput_event_gesture_get_dy_unaccelerated(m_gestureEvent));
 }
 
 bool GestureEvent::isCancelled() const