Fix glyph cache crash

[InnocentusLime]

Synopsis

Fixes #815. The Style::with_font method clones the received font and then changes it, placing a different atlas pointer in the cloned font. It, however, doesn't do anything with the characters field.

Because of that, if the original font caches more glyphs -- the font passed to Style::with_font ends up in an inconsistent state, which leads to either crashed or garbage looking graphics. This happens from very naive things like measuring text after construing a Style, so I think it's worth fixing.

The fix worked when I tested it against the example in issue #815.

The Fix

I simply modified the routine to supply the cloned font with its own characters map. To do this I also had to add a pub(crate) method for setting characters.

Alternatives

The alternative is to stop giving the cloned font a different atlas. But it looked like it might be against the original intention behind Style's design.

[KnorrFG]

Hey, I was wondering when this is getting merged. Are there any policies in place that prevent the merge?

[InnocentusLime]

@not-fl3

Is there anything preventing the merge of this PR? 🤔