The Pango.Layout objects 'first_line_layout' and 'second_line_layout'
were being assigned a colour in different ways, which could be a bit
confusing (the latter was using <span color="xyz"> markup, the former
was using Cairo.Context.SetSourceColor() API), so let's use markup
in both cases.

This commit changes behaviour, but doesn't (shouldn't) produce a
different rendering result in any way.