const PALETTE_INDEX: LocalKey<RefCell<Box<[u16]>>>;Expand description
Palette index per global id, reused across encodes on this thread. It takes 128 KiB on each thread that encodes a section, allocated on first use.
Only the slots of the palette being encoded hold an index; every other slot holds
EMPTY_SLOT, so a value missing from its palette is detected instead of being
encoded with an index left over from an earlier palette.