Separate the text shaping functionality from font metrics by having a
FontMetrics trait next to the TextShaper. AbstractFont is the combining
super trait. This allows eliminating the font height member from
TextParagraphLayout and improving the overall naming of fields and
types.
Finally, this prepares the API for composability of TextShaper for font
fallback handling.
Replace the abstract glyph trait with a glyph struct. That way the text
layout code can operate properly on a struct with fields, instead of on
functions on a trait (some of which returning a mutable reference). The
input is a glyph with offset, advance, etc. - everything needed for the
layout and the output is a position along with the platform specific
glyph data.
Remove the font reference field anymore as it's not needed anymore and
remove the manual clone implementations. Those were needed because
otherwise the TextShaper trait would have to also support clone - as
[derive(Clone)] imposes that. Now we can impose that just on Glyph and
that's easy and makes sense.
