Improve support for GFlags arguments

[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.

[jcupitt]

pyvips has this feature now with libvips/pyvips#429

We should do something similar for ruby-vips.

[jcupitt]

I finally got around to this, sorry for the delay, there's a PR here: #421

It allows things like:

image.pngsave "x.png", keep: [:icc, :xmp]
image.pngsave "x.png", keep: :icc
image.pngsave "x.png", keep: "icc xmp"
image.pngsave "x.png", keep: ["icc", "xmp"]
image.pngsave "x.png", keep: [1 << 3, 1 << 1]

Let's move any discussion there.

[jcupitt]

All done, I'll close.