The nine source or destination sub-rectangles produced by a nine-slice split.
Stored as a three-by-three grid in reading order — row 0 is the top
(TOP_LEFT, TOP, TOP_RIGHT), row 1 the middle (LEFT, CENTER,
RIGHT), row 2 the bottom (BOTTOM_LEFT, BOTTOM, BOTTOM_RIGHT).
The named accessors read through those indices, so callers get
self-documenting access while the storage stays a single array that
can be iterated directly when emitting the nine drawImage calls.
Implements named access over the three-by-three nine-slice grid.
The index constants live in this module’s const.rs; each accessor
maps a grid position to a self-documenting name so call sites and
assert messages read as geometry rather than as (row, column) pairs.