Optimize decoding of fixed huffman blocks

[fintelia]

This PR adds two optimizations to the decoding of fixed huffman blocks:

• Any fixed blocks with only an EOF symbol are skipped without building a decoding table or running the decoding loop. These should be rare in PNGs, but some encoders may write them due to performing "partial flushes" as they are feeding data into zlib.
• Consecutive fixed blocks share the same decoding table, so we don't need to rebuild it between blocks.

A possible future optimization would be to cache the fixed block decoding tables in a static variable the first time they are needed. The downside is that it would consume ~16 KB of memory for the rest of the program's lifetime, even if no more decompression was performed.