Engine: for touch-to-mouse implement a relative cursor motion

Engine/ac/sys_events.cpp

@@ -542,6 +542,19 @@ static void send_mouse_motion_event(int x, int y, int xrel, int yrel)
     SDL_PushEvent(&evt);
 }
 
+static Point mouse_pos_from_finger(const SDL_TouchFingerEvent &event,
+    const Size &logical_sz, const Point &old_pos)
+{
+    // Handle relative movement
+    if (event.dx != 0.f || event.dy != 0.f)
+    {
+        return Point(old_pos.X + event.dx * logical_sz.Width,
+                     old_pos.Y + event.dy * logical_sz.Height);
+    }
+    // Handle absolute movement
+    return Point(event.x * logical_sz.Width, event.y * logical_sz.Height);
+}
+
 static void detect_double_tap(const SDL_TouchFingerEvent &event, bool down)
 {
     auto tap_ts = AGS_Clock::now();
@@ -602,10 +615,10 @@ void on_sdl_touch_down(const SDL_TouchFingerEvent &event)
         // otherwise, ignore the movement for now
         if ((!touch_mouse_ignore_motion) && (mouse_but == SDL_BUTTON_LEFT))
         {
-            touch_mouse_pos = Point(event.x * w, event.y * h);
+            touch_mouse_pos = mouse_pos_from_finger(event, Size(w, h), touch_mouse_pos);
             touch_mouse_start_pos = touch_mouse_pos;
             touch_mouse_is_dragging = false;
-            send_mouse_motion_event(touch_mouse_pos.X, touch_mouse_pos.Y, 0, 0 /* TODO? */);
+            send_mouse_motion_event(touch_mouse_pos.X, touch_mouse_pos.Y, event.dx * w, event.dy * h);
         }
         // If more than one finger was down, lock the cursor motion,
         // and force any following emulated clicks to RMB,
@@ -698,8 +711,8 @@ void on_sdl_touch_motion(const SDL_TouchFingerEvent &event)
         int mouse_but = tfinger_to_mouse_but(event.fingerId);
         if ((!touch_mouse_ignore_motion) && (mouse_but == SDL_BUTTON_LEFT))
         {
-            touch_mouse_pos = Point(event.x * w, event.y * h);
-            send_mouse_motion_event(touch_mouse_pos.X, touch_mouse_pos.Y, 0, 0 /* TODO? */);
+            touch_mouse_pos = mouse_pos_from_finger(event, Size(w, h), touch_mouse_pos);
+            send_mouse_motion_event(touch_mouse_pos.X, touch_mouse_pos.Y, event.dx * w, event.dy * h);
             Point trigger_dist(w * touch_drag_trigger_dist, h * touch_drag_trigger_dist);
             if (DistanceBetween(touch_mouse_pos, touch_mouse_start_pos) >= touch_drag_trigger_dist)
             {