LibPDF: Add string drawing code for Type0Fonts

[nico]

This is very similar to SimpleFont::draw_string() for now, but it'll become a bit different when we add support for vertical text.

CIDFontType now only needs to draw single glyphs. Neither of the subclasses can do that yet, so no behavior change yet.

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More scaffolding for type0 fonts.

[nico]

Red CI unrelated.