Fallback font handling

[RobertMueller2]

Right now, entering characters not in the specified font result in tofu or nothing.

Femtovg itself is capable of using fallback fonts, as shown in at least two of femtovg's examples. So it should be possible to improve this in Satty.

This might or might not work better if fallback fonts can be user configured. If so, we also need a sensible default configuration. And perhaps consider shipping suitable fonts.