Add stylistic font transformation: italic

[pelson]

We might get away with an automatic transformation for most of the characters - but there are plenty of examples in the XKCD back-catalogue of actual italicised versions (fans: please provide references to the XKCDs in the comments).

Useful guide: https://fontforge.org/docs/ui/dialogs/Styles.html

[pelson]

@Tim-the-gitter wrote in #43:

And what about italics? Maybe this, this, this, this

[dummy-index]

(From another perspective, there are combinations where the variation in stem slant is currently concerning, such as 'dB'. Ideally, we would like to classify the outlines extracted from handwriting_minimal.png by slant. That alone won't gather enough glyphs to create an italic font, but still.)