This is mainly just for kicks and giggles, since it really won't
work quite properly anyway, due to the lack of grapheme indexing.
The next real thing is to... well, start from scratch.
They were two pretty much identical bugs in both index_offset_vertical_v2d()
and index_set_horizontal_v2d(). Neither method was checking for slipping
off the end of a line block.
The formatters now work on grapheme iterators instead of directly on
lines, which frees up the LineFormatter to break up long lines into
smaller blocks of text. This is partially taken advantage of right
now in various parts of the code, but more work is still needed to
get it both working properly and fast.
This is a big shift in the formatter API's. It simplifies the
responsibilities of the implementers so that pretty much all they
have to do it implement an iterator. Everything else will be
automatically derived from that. (Or, at least, that's the hope.)
