Allow to selectively use NSGLView Canvas

Various GdkEvents are not yet handled correctly, eg. unpacking a widget
from its container does not unmap it, nor are remaining widgets in the
contained re-positioned (size allocation does not change, nor does the
mapping). This affects eg. Mixbus Strips

libs/canvas/canvas.cc

@@ -401,7 +401,13 @@ GtkCanvas::GtkCanvas ()
 	add_events (Gdk::BUTTON_PRESS_MASK | Gdk::BUTTON_RELEASE_MASK | Gdk::POINTER_MOTION_MASK |
 		    Gdk::SCROLL_MASK | Gdk::ENTER_NOTIFY_MASK | Gdk::LEAVE_NOTIFY_MASK |
 		    Gdk::KEY_PRESS_MASK | Gdk::KEY_RELEASE_MASK);
+}
 
+void
+GtkCanvas::use_nsglview ()
+{
+	assert (!_nsglview);
+	assert (!is_realized());
 #ifdef ARDOUR_CANVAS_NSVIEW_TAG // patched gdkquartz.h
 # ifndef __ppc__ // would need to flip RGBA <> RGBA
 	_nsglview = Gtkmm2ext::nsglview_create (this);

libs/canvas/canvas/canvas.h

@@ -181,6 +181,8 @@ public:
 	GtkCanvas ();
 	~GtkCanvas () { _in_dtor = true ; }
 
+	void use_nsglview ();
+
 	void request_redraw (Rect const &);
 	void request_size (Duple);
 	void grab (Item *);