surface: Add coordinate translation

We maintain offsets for popups, so we can translate
coordinates between surfaces that are attached directly
or indirectly to the same toplevel. Add an api for that.

gdk/gdksurface.c

@@ -4076,3 +4076,41 @@ gdk_surface_handle_event (GdkEvent *event)
 
   return handled;
 }
+
+gboolean
+gdk_surface_translate_coordinates (GdkSurface *from,
+                                   GdkSurface *to,
+                                   double     *x,
+                                   double     *y)
+{
+  int x1, y1, x2, y2;
+  GdkSurface *f, *t;
+
+  x1 = 0;
+  y1 = 0;
+  f = from;
+  while (f->parent)
+    {
+      x1 += f->x;
+      y1 += f->y;
+      f = f->parent;
+    }
+
+  x2 = 0;
+  y2 = 0;
+  t = to;
+  while (t->parent)
+    {
+      x2 += t->x;
+      y2 += t->y;
+      t = t->parent;
+    }
+
+  if (f != t)
+    return FALSE;
+
+  *x += x1 - x2;
+  *y += y1 - y2;
+
+  return TRUE;
+}

gdk/gdksurface.h

@@ -557,6 +557,12 @@ gint          gdk_surface_get_origin     (GdkSurface      *surface,
                                           gint            *x,
                                           gint            *y);
 GDK_AVAILABLE_IN_ALL
+gboolean gdk_surface_translate_coordinates (GdkSurface *from,
+                                            GdkSurface *to,
+                                            double     *x,
+                                            double     *y);
+
+GDK_AVAILABLE_IN_ALL
 void          gdk_surface_get_frame_extents (GdkSurface     *surface,
                                              GdkRectangle  *rect);