fix: allow compression=0 for pngsave - #101
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cshum
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Sep 7, 2026
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vipsgen_set_int skips zero values, so the Go zero value falls back to the libvips default. pngsave compression is 0-9 with default 6, so Compression: 0 currently gets silently dropped and you end up with level 6. There's no way to ask for an uncompressed PNG. Same issue as effort=0 in #95.
This whitelists compression for the pngsave family (pngsave, pngsave_buffer, pngsave_target) to use vipsgen_set_int_allow_zero.
Checked the libvips registrations on 8.16.1 / 8.17.3 / 8.18.6, ranges and defaults are the same across the CI matrix.
Not whitelisting params where 0 is an index or an auto marker (jpegsave quant_table, tiffsave/ppmsave bitdepth, jxlsave tier), since explicit 0 is indistinguishable from unset there. pngsave effort stays out too, its range is 1-10.
Snapshot tests cover both directions: pngsave compression uses allow_zero, while pngsave effort and dzsave compression keep the zero-skip.
Also renamed shouldAllowZeroEffort to shouldIntAllowZero since it's no longer effort-specific.
One thing I wasn't sure about: dzsave compression (range -1..9, default 0). 0 means "store" there as well, but since 0 is also the default, the zero-skip makes no difference today. Could add it for consistency so that compression=0 means "no compression" everywhere, but left it out to keep this minimal.