Keep shuttle ctrl in sync with actual speed

When using "VS" the numeric control shows the "default speed"
when not rolling. Then when starting to roll default speed will
be the actual speed.

Previously the shuttle was not updated when `last_speed_displayed`
was the current speed. This dated back to before "VS" showed
default speed.

gtk2_ardour/shuttle_control.cc

@@ -155,6 +155,7 @@ ShuttleControl::ShuttleControl ()
 	shine_pattern         = 0;
 	last_shuttle_request  = 0;
 	last_speed_displayed  = -99999999;
+	last_shuttle_fract    = -99999999;
 	shuttle_grabbed       = false;
 	shuttle_speed_on_grab = 0;
 	shuttle_fract         = 0.0;
@@ -318,13 +319,6 @@ ShuttleControl::map_transport_state ()
 		speed = _session->actual_speed ();
 	}
 
-	if ((fabsf (speed - last_speed_displayed) < 0.005f) // dead-zone
-	    && !(speed == 1.f && last_speed_displayed != 1.f)
-	    && !(speed == 0.f && last_speed_displayed != 0.f)) {
-		return; // nothing to see here, move along.
-	}
-
-	// Q: is there a good reason why we  re-calculate this every time?
 	if (fabs (speed) <= (2 * DBL_EPSILON)) {
 		shuttle_fract = 0;
 	} else {
@@ -335,6 +329,11 @@ ShuttleControl::map_transport_state ()
 		}
 	}
 
+	if ((fabsf (shuttle_fract - last_shuttle_fract) < 0.005f)) {
+		/* dead-zone */
+		return;
+	}
+
 	queue_draw ();
 }
 
@@ -725,6 +724,7 @@ ShuttleControl::render (Cairo::RefPtr<Cairo::Context> const& ctx, cairo_rectangl
 	}
 
 	last_speed_displayed = actual_speed;
+	last_shuttle_fract = shuttle_fract;
 
 	_info_button.set_text (buf);
 
@@ -765,6 +765,7 @@ ShuttleControl::parameter_changed (std::string p)
 	} else if (p == "shuttle-max-speed") {
 		shuttle_max_speed    = Config->get_shuttle_max_speed ();
 		last_speed_displayed = -99999999;
+		last_shuttle_fract = -99999999;
 		map_transport_state ();
 		use_shuttle_fract (true);
 		delete shuttle_context_menu;

gtk2_ardour/shuttle_control.h

@@ -111,6 +111,7 @@ protected:
 	bool                                   _hovering;
 	float                                  shuttle_max_speed;
 	float                                  last_speed_displayed;
+	float                                  last_shuttle_fract;
 	bool                                   shuttle_grabbed;
 	double                                 shuttle_speed_on_grab;
 	double                                 requested_speed;