allow runtime control over cairo save/restore around item rendering

This is to allow easier debugging of whether a given item's render method fails to leave a cairo
context in the same state it received it in

libs/canvas/canvas.cc

@@ -68,6 +68,13 @@ Canvas::Canvas ()
 #else
 	_use_intermediate_surface = NULL != g_getenv("ARDOUR_INTERMEDIATE_SURFACE");
 #endif
+
+	if (g_getenv ("ARDOUR_ITEM_CAIRO_SAVE_RESTORE")) {
+		item_save_restore = true;
+	} else {
+		item_save_restore = false;
+	}
+
 	set_epoch ();
 }
 

libs/canvas/canvas/canvas.h

@@ -188,6 +188,9 @@ public:
 
 	void set_debug_render (bool yn) { _debug_render = yn; }
 	bool debug_render() const { return _debug_render; }
+
+	bool item_save_restore;
+
 protected:
 	Root             _root;
 	uint32_t         _queue_draw_frozen;

libs/canvas/item.cc

@@ -941,8 +941,13 @@ Item::render_children (Rect const & area, Cairo::RefPtr<Cairo::Context> context)
 					}
 				}
 #endif
-
+				if (_canvas->item_save_restore) {
+					context->save();
+				}
 				(*i)->render (area, context);
+				if (_canvas->item_save_restore) {
+					context->restore();
+				}
 				++render_count;
 			}