Towards a consistent render() API.

Towards a consistent render() API.

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.