[smooth] Clean up the null cell usage.
Put the null cell at the end of the pool and store it explicitly so that
we can use it as both the limit and the dumpster.
* src/smooth/ftgrays.c (gray_TWorker): Store the last `cell_null` and
remove unnecesary fields.
(NULL_CELL_PTR, CELL_IS_NULL): Remove in favor of explicit `cell_null`.
(gray_dump_cells, gray_set_cell, gray_sweep{,_direct}): Update callers.
(gray_convert_glyph_inner): Trace remaining cells (oh well).
(gray_convert_glyph): Set up `cell_null` and slightly improve the pool
management.
1 file changed
