Use a dedicated Canvas Group for MRV

This fixes rendering of opaque MIDI regions (previously
MIDI regions were always transparent). This change provides a
way to "flatten" layered MIDI regions, while still allowing
to show the note-line and grid behind the regions.

gtk2_ardour/editor_drag.cc

@@ -1992,7 +1992,7 @@ RegionMoveDrag::finished_no_copy (
 			   No need to do anything for copies as they are fake regions which will be deleted.
 			*/
 
-			rv->get_canvas_group()->reparent (dest_rtv->view()->canvas_item());
+			rv->get_canvas_group()->reparent (dest_rtv->view()->region_canvas());
 			rv->get_canvas_group()->set_y_position (i->initial_y);
 			rv->drag_end ();
 
@@ -2190,7 +2190,7 @@ RegionMotionDrag::aborted (bool)
 		TimeAxisView* tv = &(rv->get_time_axis_view ());
 		RouteTimeAxisView* rtv = dynamic_cast<RouteTimeAxisView*> (tv);
 		assert (rtv);
-		rv->get_canvas_group()->reparent (rtv->view()->canvas_item());
+		rv->get_canvas_group()->reparent (rtv->view()->region_canvas());
 		rv->get_canvas_group()->set_y_position (0);
 		rv->drag_end ();
 		rv->move (-_total_x_delta, 0);
@@ -2248,7 +2248,7 @@ RegionInsertDrag::finished (GdkEvent * event, bool)
 
 	RouteTimeAxisView* dest_rtv = dynamic_cast<RouteTimeAxisView*> (_time_axis_views[pos]);
 
-	_primary->get_canvas_group()->reparent (dest_rtv->view()->canvas_item());
+	_primary->get_canvas_group()->reparent (dest_rtv->view()->region_canvas());
 	_primary->get_canvas_group()->set_y_position (0);
 
 	boost::shared_ptr<Playlist> playlist = dest_rtv->playlist();

gtk2_ardour/ghostregion.cc

@@ -208,7 +208,7 @@ MidiGhostRegion::MidiGhostRegion(MidiRegionView& rv,
                                  TimeAxisView& source_tv,
                                  double initial_unit_pos)
 	: GhostRegion (rv,
-	               msv.midi_underlay_group,
+	               msv.midi_underlay(),
 	               msv.trackview(),
 	               source_tv,
 	               initial_unit_pos)

gtk2_ardour/midi_region_view.cc

@@ -3687,9 +3687,9 @@ MidiRegionView::note_mouse_position (float x_fraction, float /*y_fraction*/, boo
 uint32_t
 MidiRegionView::get_fill_color() const
 {
-	const std::string mod_name = (_dragging ? "dragging region" :
+	const std::string mod_name = _dragging ? "dragging region" :
 	                              trackview.editor().internal_editing() ? "editable region" :
-	                              "midi frame base");
+	                               (_region->opaque() && !_region->muted ()) ? "opaque region base" : "transparent region base";
 
 
 	if (_selected) {

gtk2_ardour/midi_streamview.cc

@@ -74,9 +74,15 @@ MidiStreamView::MidiStreamView (MidiTimeAxisView& tv)
 	, _note_lines (0)
 	, _updates_suspended (false)
 {
+
 	/* use a group dedicated to MIDI underlays. Audio underlays are not in this group. */
-	midi_underlay_group = new ArdourCanvas::Container (_canvas_group);
+	_midi_underlay = new ArdourCanvas::Container (_canvas_group);
-	midi_underlay_group->lower_to_bottom();
+	_midi_underlay->lower_to_bottom();
+
+	/* use a dedicated group for MIDI regions (on top of the grid and lines) */
+	_region_group = new ArdourCanvas::Container (_canvas_group);
+	_region_group->raise_to_top ();
+	_region_group->set_render_with_alpha (UIConfiguration::instance().modifier ("region alpha").a());
 
 	/* put the note lines in the timeaxisview's group, so it
 	   can be put below ghost regions from MIDI underlays
@@ -117,11 +123,11 @@ MidiStreamView::create_region_view (boost::shared_ptr<Region> r, bool /*wfd*/, b
 	RegionView* region_view = NULL;
 	if (recording) {
 		region_view = new MidiRegionView (
-			_canvas_group, _trackview, region,
+			_region_group, _trackview, region,
 			_samples_per_pixel, region_color, recording,
 			TimeAxisViewItem::Visibility(TimeAxisViewItem::ShowFrame));
 	} else {
-		region_view = new MidiRegionView (_canvas_group, _trackview, region,
+		region_view = new MidiRegionView (_region_group, _trackview, region,
 		                                  _samples_per_pixel, region_color);
 	}
 
@@ -579,6 +585,7 @@ MidiStreamView::setup_rec_box ()
 void
 MidiStreamView::color_handler ()
 {
+	_region_group->set_render_with_alpha (UIConfiguration::instance().modifier ("region alpha").a());
 	draw_note_lines ();
 
 	if (_trackview.is_midi_track()) {

gtk2_ardour/midi_streamview.h

@@ -72,7 +72,6 @@ public:
 	};
 
 	Gtk::Adjustment note_range_adjustment;
-	ArdourCanvas::Container* midi_underlay_group;
 
 	void set_note_range(VisibleNoteRange r);
 
@@ -114,10 +113,16 @@ public:
 	void suspend_updates ();
 	void resume_updates ();
 
+	ArdourCanvas::Container* midi_underlay () const { return _midi_underlay; }
+	ArdourCanvas::Container* region_canvas () const { return _region_group; }
+
 protected:
 	void setup_rec_box ();
 	void update_rec_box ();
 
+	ArdourCanvas::Container* _midi_underlay;
+	ArdourCanvas::Container* _region_group;
+
 private:
 
 	RegionView* add_region_view_internal (

gtk2_ardour/streamview.h

@@ -86,7 +86,7 @@ public:
 	virtual bool can_change_layer_display() const { return true; }
 	LayerDisplay layer_display () const { return _layer_display; }
 
-	ArdourCanvas::Container* canvas_item() { return _canvas_group; }
+	virtual ArdourCanvas::Container* region_canvas () const { return _canvas_group; }
 
 	enum ColorTarget {
 		RegionColor,