These are presets for lossless coding, similar to zlib.
The shortcut for lossless coding is now, e.g.:
cwebp -z 5 in.png -o out_lossless.webp
There are 10 possible values for -z parameter:
0 (fastest, lowest compression)
to 9 (slowest, best compression)
A reasonable tradeoff is -z 6, e.g.
-z 9 can be quite slow, so use with care.
This -z option is just a shortcut for some pre-defined
'-lossless -m xx -q yy' combinations.
Change-Id: I6ae716456456aea065469c916c2d5ca4d6c6cf04
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