For normal cairo-widgets ardour should not use image surfaces on macOS,
because that bypasses retina scaling. In theory explicit upsampling could
be performed (compere to openGL/cairo), but for the case at hand that is
overkill and inconvenient.
Performance critical widgets that render periodically can enable openGL
backing. Casual widgets (buttons with text, knobs, sliders etc) can be
rendered directly without any significant performance penalty.
