Introduces `fill`, which fills between two `Fraction`s, use this instead
of `yHalfs` and friends wherever they're used, which also means we can
remove `rect`.
This commit does change alignment of the vertical/horizontal eighths in
certain cell sizes, but the change is for the better IMO. Also changes
the center-point alignment of smooth mosaics for odd cell widths, but
the change is no more than half a pixel at worst and is probably an
improvement ultimately.
c838d3d7d2