LibGfx: Use ZlibCompressor for compressing PNG files

Previously we were hand-writing all the Zlib and raw Deflated data
structures here, but now PNGs will be compressed using ZlibCompressor
which will actually try to compress something! :^)

Note that we don't do any filtering that should help compress data even
more, but even now the results are pretty good:

- screenshots of my Serenity desktop are take now about 55 KiB, where
  previously it was 3 MiB.
- re-encoding NASA photo of the Earth[1] to PNG shows a 2x improvement
  (34.3 MiB -> 16.5 MiB).

[1]: https://commons.wikimedia.org/wiki/File:The_Blue_Marble_(remastered).jpg

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