Before things would be cast to u16 earlier than needed, which would
cause problems for insanely long lines (longer than 2^16 ~ 65 thousand)

Better performance, and otherwise very long lines with lots of tabs
will wrap around the u16 and come back on the other side, messing up
the beginning skip_levels.

Instead of repeatedly checking if it is in_bounds, calculate the
max_indent beforehand and just loop. I added a debug_assert to "prove"
that it never tries drawing out of bounds.