Since 5.8-245-g3e43585fa, a response hides the dialog Window
in ArdourDialog::on_response (to prevent dialogs windows staying
around unresponsively while Ardour does background work).
This logic does not apply to Dialog Windows which implement
ProgressReporter or support dialog responses other than OK, Close.
This fixes an -Woverloaded-virtual ambiguity introduced in b5e613d45
void render (cairo_t*, cairo_rectagle*)
void render (Cairo::RefPtr<Cairo::Context> const&, cairo_rectangle_t*)
ArdourCanvas prefers cairomm and CairoWidget itself uses Cairo::Context,
this improves overall API consistency.
