How to use pngsave's filter option (Vips::ForeignPngFilter)

[jcupitt]

Discussed in #358

^{Originally posted by tonymarklove January 25, 2023}
After digging into the ruby-vips code I can't see anywhere that Vips::ForeignPngFilter is defined.

After some investigation it appears that the PNG saving functions in Ruby accept the values as defined in the C enum eg.

image.pngsave('file.png', filter: 0xF8)

Am I missing a simpler way to do this? Or are these filter constants missing from ruby-vips?

[jcupitt]

See discussion for possible API.