extend MidiViewBackground API

1. provide access to an EditingContext for use in piano roll headers
2. provide pre-computed note positions and heights for use in
   piano roll headers

gtk2_ardour/midi_streamview.cc

@@ -64,7 +64,7 @@ using namespace Editing;
 
 MidiStreamView::MidiStreamView (MidiTimeAxisView& tv)
 	: StreamView (tv)
-	, MidiViewBackground (_canvas_group)
+	, MidiViewBackground (_canvas_group, tv.editor())
 	, _updates_suspended (false)
 {
 	/* use a dedicated group for MIDI regions (on top of the grid and lines) */

gtk2_ardour/midi_view_background.cc

@@ -135,7 +135,17 @@ MidiViewBackground::update_contents_height ()
 }
 
 void
-MidiViewBackground::draw_note_lines()
+MidiViewBackground::get_note_positions (std::vector<int>& numbers, std::vector<int>& pos, std::vector<int>& heights) const
+{
+	for (auto const & r : _note_lines->rects()) {
+		numbers.push_back (r.index);
+		pos.push_back (r.y0);
+		heights.push_back (r.height());
+	}
+}
+
+void
+MidiViewBackground::setup_note_lines()
 {
 	if (updates_suspended()) {
 		return;

gtk2_ardour/midi_view_background.h

@@ -44,6 +44,8 @@ namespace ArdourCanvas {
 	class RectSet;
 }
 
+class EditingContext;
+
 /** A class that provides various context for a MidiVieww:
         = note ranges
         * color information
@@ -53,7 +55,7 @@ namespace ArdourCanvas {
 class MidiViewBackground : public virtual ViewBackground
 {
   public:
-	MidiViewBackground (ArdourCanvas::Item* parent);
+	MidiViewBackground (ArdourCanvas::Item* parent, EditingContext& ec);
 	~MidiViewBackground ();
 
 	Gtk::Adjustment note_range_adjustment;
@@ -125,7 +127,12 @@ class MidiViewBackground : public virtual ViewBackground
 
 	virtual ARDOUR::InstrumentInfo* instrument_info() const = 0;
 
+	void get_note_positions (std::vector<int>& numbers, std::vector<int>& pos, std::vector<int>& heights) const;
+
+	EditingContext& editing_context() const { return _editing_context; }
+
   protected:
+	EditingContext&           _editing_context;
 	bool                      _range_dirty;
 	double                    _range_sum_cache;
 	uint8_t                   _lowest_note;   ///< currently visible

gtk2_ardour/pianoroll_background.cc

@@ -25,7 +25,7 @@
 #include "midi_view.h"
 
 PianorollMidiBackground::PianorollMidiBackground (ArdourCanvas::Item* parent, Pianoroll& pr)
-	: MidiViewBackground (parent)
+	: MidiViewBackground (parent, pr)
 	, view (nullptr)
 	, pianoroll (pr)
 	, _width (0.)