VipsForeignPngFilter

flags Vips.ForeignPngFilter

The values mirror those of png.h in libpng.

Name	Description
`VIPS_FOREIGN_PNG_FILTER_NONE`	No filtering.
`VIPS_FOREIGN_PNG_FILTER_SUB`	Difference to the left.
`VIPS_FOREIGN_PNG_FILTER_UP`	Difference up.
`VIPS_FOREIGN_PNG_FILTER_AVG`	Average of left and up.
`VIPS_FOREIGN_PNG_FILTER_PAETH`	Pick best neighbor predictor automatically.
`VIPS_FOREIGN_PNG_FILTER_ALL`	Adaptive.