Add API to set openGL backing scale

libs/canvas/canvas.cc

@@ -555,12 +555,12 @@ GtkCanvas::GtkCanvas ()
 }
 
 void
-GtkCanvas::use_nsglview ()
+GtkCanvas::use_nsglview (bool retina)
 {
 	assert (!_nsglview);
 	assert (!get_realized());
 #ifdef ARDOUR_CANVAS_NSVIEW_TAG // patched gdkquartz.h
-	_nsglview = Gtkmm2ext::nsglview_create (this);
+	_nsglview = Gtkmm2ext::nsglview_create (this, retina);
 #endif
 }
 

libs/canvas/canvas/canvas.h

@@ -217,7 +217,7 @@ public:
 	GtkCanvas ();
 	~GtkCanvas () { _in_dtor = true ; }
 
-	void use_nsglview ();
+	void use_nsglview (bool retina = true);
 
 	void request_redraw (Rect const &);
 	void request_size (Duple);

libs/gtkmm2ext/cairo_widget.cc

@@ -96,13 +96,13 @@ CairoWidget::set_canvas_widget ()
 }
 
 void
-CairoWidget::use_nsglview ()
+CairoWidget::use_nsglview (bool retina)
 {
 	assert (!_nsglview);
 	assert (!_canvas_widget);
 	assert (!get_realized());
 #ifdef ARDOUR_CANVAS_NSVIEW_TAG // patched gdkquartz.h
-	_nsglview = Gtkmm2ext::nsglview_create (this);
+	_nsglview = Gtkmm2ext::nsglview_create (this, retina);
 #endif
 }
 

libs/gtkmm2ext/gtkmm2ext/cairo_widget.h

@@ -41,7 +41,7 @@ public:
 	virtual ~CairoWidget ();
 
 	void set_canvas_widget ();
-	void use_nsglview ();
+	void use_nsglview (bool retina = true);
 	void use_image_surface (bool yn = true);
 
 	/* swizzle Gtk::Widget methods for Canvas::Widget */

libs/gtkmm2ext/gtkmm2ext/nsglview.h

@@ -25,7 +25,6 @@ namespace Gtkmm2ext
 {
 	class CairoCanvas;
 
-	void* nsglview_create (CairoCanvas*);
 	void* nsglview_create (CairoCanvas*, bool use_backing_scale);
 	void  nsglview_overlay (void*, GdkWindow*);
 	void  nsglview_resize (void*, int x, int y, int w, int h);

libs/gtkmm2ext/nsglview.mm

@@ -374,12 +374,6 @@ __attribute__ ((visibility ("hidden")))
 }
 @end
 
-void*
-Gtkmm2ext::nsglview_create (Gtkmm2ext::CairoCanvas* canvas)
-{
-	return nsglview_create (canvas, true);
-}
-
 void*
 Gtkmm2ext::nsglview_create (Gtkmm2ext::CairoCanvas* canvas, bool use_backing_scale)
 {