Fix printing under Windows

For some reason the Scintilla widget's Pango context don't have a
resolution set on Windows, so we get an invalid one.  Fix this by
correctly peek the resolution from the map if the context doesn't
have one by itself.

https://developer.gnome.org/pango/stable/pango-Cairo-Rendering.html#pango-cairo-context-get-resolution

Thanks to Matthew Brush for debugging this!

Closes #961.