Release NSGLView, fixes heap-use-after-free

libs/canvas/canvas.cc

@@ -562,6 +562,16 @@ GtkCanvas::GtkCanvas ()
 	            Gdk::KEY_PRESS_MASK | Gdk::KEY_RELEASE_MASK);
 }
 
+GtkCanvas::~GtkCanvas ()
+{
+	_in_dtor = true;
+#ifdef __APPLE__
+	if (_nsglview) {
+		Gtkmm2ext::nsglview_destroy (_nsglview);
+	}
+#endif
+}
+
 void
 GtkCanvas::set_single_exposure (bool yn)
 {

libs/canvas/canvas/canvas.h

@@ -218,7 +218,7 @@ class LIBCANVAS_API GtkCanvas : public Canvas, public Gtk::EventBox, public Gtkm
 {
 public:
 	GtkCanvas ();
-	~GtkCanvas () { _in_dtor = true ; }
+	~GtkCanvas ();
 
 	void use_nsglview (bool retina = true);
 

libs/gtkmm2ext/gtkmm2ext/nsglview.h

@@ -26,9 +26,11 @@ namespace Gtkmm2ext
 	class CairoCanvas;
 
 	void* nsglview_create (CairoCanvas*, bool use_backing_scale);
+	void  nsglview_destroy (void*);
 	void  nsglview_overlay (void*, GdkWindow*);
 	void  nsglview_resize (void*, int x, int y, int w, int h);
 	void  nsglview_queue_draw (void*, int x, int y, int w, int h);
 	void  nsglview_set_visible (void*, bool);
+
 }
 #endif

libs/gtkmm2ext/nsglview.mm

@@ -396,6 +396,14 @@ Gtkmm2ext::nsglview_create (Gtkmm2ext::CairoCanvas* canvas, bool use_backing_sca
 	return gl_view;
 }
 
+void
+Gtkmm2ext::nsglview_destroy (void* glv)
+{
+	ArdourCanvasOpenGLView* gl_view = (ArdourCanvasOpenGLView*) glv;
+	[gl_view removeFromSuperview];
+	[gl_view release];
+}
+
 void
 Gtkmm2ext::nsglview_overlay (void* glv, GdkWindow* window)
 {