colorscale: Redraw correct widget after setting rgba

The entire color scale hack is still done in GtkRange, which draws the
color scale in the range gizmo. So, to correctly redraw the color scale
when setting a new color, we need to redraw the proper widget and that's
the trough widget.

Fixes #1453

gtk/gtkcolorscale.c

@@ -289,7 +289,7 @@ gtk_color_scale_set_rgba (GtkColorScale *scale,
   GtkColorScalePrivate *priv = gtk_color_scale_get_instance_private (scale);
 
   priv->color = *color;
-  gtk_widget_queue_draw (GTK_WIDGET (scale));
+  gtk_widget_queue_draw (gtk_range_get_trough_widget (GTK_RANGE (scale)));
 }
 
 GtkWidget *

gtk/gtkrange.c

@@ -2948,6 +2948,14 @@ gtk_range_get_slider_widget (GtkRange *range)
   return priv->slider_widget;
 }
 
+GtkWidget *
+gtk_range_get_trough_widget (GtkRange *range)
+{
+  GtkRangePrivate *priv = gtk_range_get_instance_private (range);
+
+  return priv->trough_widget;
+}
+
 void
 gtk_range_start_autoscroll (GtkRange      *range,
                             GtkScrollType  scroll_type)

gtk/gtkrangeprivate.h

@@ -42,6 +42,8 @@ gint               _gtk_range_get_stop_positions           (GtkRange      *range
                                                             gint         **values);
 
 GtkWidget         *gtk_range_get_slider_widget             (GtkRange *range);
+GtkWidget         *gtk_range_get_trough_widget             (GtkRange *range);
+
 
 void               gtk_range_start_autoscroll              (GtkRange      *range,
                                                             GtkScrollType  scroll_type);