`CPU_HUFFMAN_ZLIB` Compressor Doesn't Use Huffman Coding

[ben-e-whitney]

When the lossless compressor is set to CPU_HUFFMAN_ZLIB in the header, Huffman coding is not applied to the quantized coefficients before they are passed to compress_memory_z. See this case. We should rename the enumeration value to CPU_ZLIB and add a case where both Huffman coding and compress_memory_z are applied.