fix crash when cleaning up canvas items

fix crash when cleaning up canvas items

Backport from Mixbus. In Ardour this does not currently matter
since there is no top-level canvas destroyed top-down.

...
Editor::session_going_away
MixerStrip::~MixerStrip
MixbusStripCanvas::~MixbusStripCanvas
ArdourCanvas::GtkCanvas::~GtkCanvas
ArdourCanvas::Canvas::~Canvas
ArdourCanvas::Root::~Root
ArdourCanvas::Container::~Container
ArdourCanvas::Item::~Item
ArdourCanvas::Item::clear_items
GtkCanvas::item_going_away()
queue_draw_item_area()

..expose can take place async to garbage collection: crash in one of the
items.