Issue #1531 - Zooming with mouse movement should keep track of original point

When Control-Button2-Zooming, remember the start point, pass it to
gimp_display_shell_scale_drag() and force gimp_display_shell_scale()
to zoom around that point by passing GIMP_ZOOM_FOCUS_POINTER and
faking the point using gimp_display_shell_push_zoom_focus_pointer_pos().

(cherry picked from commit 792cd581a2)

app/display/gimpdisplayshell.h

@@ -199,6 +199,8 @@ struct _GimpDisplayShell
   const gchar       *space_shaded_tool;
 
   gboolean           scrolling;
+  gint               scroll_start_x;
+  gint               scroll_start_y;
   gint               scroll_last_x;
   gint               scroll_last_y;
   gboolean           rotating;