Wasted cycles rebuilding decoding tables for static huffman blocks

[fintelia]

So far I've assumed that PNGs rarely use static huffman blocks because the symbol distribution is very different from other sorts of compressed data. As a result, there has been very little effort to make handling of static blocks fast. The most significant overhead is that table building is repeated every time a static block is encountered.

At a minimum, table building should be skipped for empty static huffman blocks (which don't make a ton of sense in a PNG, but are produced by certain kinds of zlib flush operations). Further improvements could be made by reusing decoding tables between consecutive static huffman blocks or even building the tables once and then caching them in a static variable.

[Shnatsel]

Is this fixed by #39 ?