Use the term glyph cluster instead of grapheme where we're dealing with
the glyphs for line breaking. Those may coincide with grapheme
boundaries, but aren't required to.
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.
This is needed in the future to be able to roll back state within the
line breaking when we can't fit even a single word into the line and
have to go back to fitting glyph by glyph.
For the unicode line break iterator the clone is a little more complicated.
The iterator is clone, but the type is anonymous. I first had an
implementation to moved the iterator into a closure, which was cloned
via another control closure. But that's complicated and still involves
allocating the various bits & pieces the closure captures.
So instead the simpler solution used here is to allocate the breaks
into a shared vector.
The unicode linebreak iterator always produces a mandatory break at end
of text. This is not only confusing, but also inconsistent with the
simple line break iterator and with trailing spaces it could cause the
line break algorithm to emit empty lines at the end of text.
