re-enable pango, use fontconfig and floats for drawImage()

- Pango seems to work fine for me (what's the issue its supposed to cause with freetype?) and means I can have fallback fonts, better kerning etc.

- Fontconfig enables finding fonts when using Pango (see also https://github.com/Automattic/node-canvas/issues/266#issuecomment-66109167).

- I have a need for floats rather than ints for the drawImage() arguments.