Fix videotimeline - initialize height

Previously this worked since rulers were initialized first, setting
the videtimeline's height. After the ruler overhaul the
videotl_bar_height was not set until after the ruler
is shown. This lead to requesting some insanely large video frame...

gtk2_ardour/editor.cc

@@ -269,6 +269,7 @@ Editor::Editor ()
 	, samples_ruler (0)
 	, minsec_ruler (0)
 	, visible_timebars (0)
+	, videotl_bar_height (4)
 	, editor_ruler_menu (0)
 	, tempo_bar (0)
 	, meter_bar (0)