This update brings several parameterization and usage flexibility
improvements on Zopfli, allowing users to choose to limit execution time
by a number of iterations without improvement, and exposing a more
advanced `ZlibEncoder` struct to tune compression block sizes and
DEFLATE block types. Some minor microoptimizations were also made.
For now, I don't expect this PR to substantially affect how OxiPNG
compresses images using its Zopfli mode, but the additional parameter
customization may come in handy for future work improving how Zopfli is
used in OxiPNG.
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