Constant LOOKBACK_SIZE 

const LOOKBACK_SIZE: usize = 32768;

PNG spec says that “deflate/inflate compression with a sliding window (which is an upper bound on the distances appearing in the deflate stream) of at most 32768 bytes”.

fdeflate requires that we keep this many most recently decompressed bytes in the out_buffer - this allows referring back to them when handling “length and distance codes” in the deflate stream).