use wheel-equivalent constraint logic for pinch zoom

Author: nevotgnab

src/views/darkroom.c

@@ -3748,8 +3748,8 @@ gboolean gesture_pinch(dt_view_t *self,
                        const int state)
 {
   dt_develop_t *dev = self->data;
-  (void)state;
   if(!dev) return FALSE;
+  const gboolean constrained = !dt_modifier_is(state, GDK_CONTROL_MASK);
 
   static double pinch_last_scale = 0.0;
 
@@ -3771,12 +3771,12 @@ gboolean gesture_pinch(dt_view_t *self,
 
   if(scale > pinch_last_scale)
   {
-    dt_dev_zoom_move(&dev->full, DT_ZOOM_SCROLL, 0.0f, 1, x, y, TRUE);
+    dt_dev_zoom_move(&dev->full, DT_ZOOM_SCROLL, 0.0f, 1, x, y, constrained);
     pinch_last_scale = scale;
   }
   else if(scale < pinch_last_scale)
   {
-    dt_dev_zoom_move(&dev->full, DT_ZOOM_SCROLL, 0.0f, 0, x, y, TRUE);
+    dt_dev_zoom_move(&dev->full, DT_ZOOM_SCROLL, 0.0f, 0, x, y, constrained);
     pinch_last_scale = scale;
   }