Go back to add_to_cairo_context rather than show_in_cairo_context.

Go back to add_to_cairo_context rather than show_in_cairo_context.

On Linux, at least, doing

add_to_cairo_context()
fill()
add_to_cairo_context()
stroke()

gives a nicer output than

show_in_cairo_context()

It's not clear exactly what the difference is, but the anti aliasing
looks better and the font outlines basically look smoother.

May help with #1815.