Fix Image.Pixel.to_pixel/3 returning gamma encoded colors and 0..255 greys for scRGB images

[hlindset]

Breaking

Returns linear light for an :scrgb image. scRGB is linear, but the pixel was resolved with Color.SRGB, so every color between black and white came out too bright and desaturated. It now resolves with Color.RGB in the :SRGB working space. Affects every function that resolves a color against an scRGB image.

Before fix. rebeccapurple came out as [170, 124, 203] (in sRGB) after being read back:

After fix. Reads back as [102, 51, 153]:

Returns relative luminance for a single band :scrgb image. Single band scRGB images were given a 0..255 integer. :white resolved to 255 instead of 1.0, and "#808080" to 137 instead of 0.216. They now resolve to CIE Y, matching how libvips collapses color to grey¹.

Footnotes

1. https://www.libvips.org/API/current/libvips-colour.html ↩

[kipcole9]

Nice catch. Should we use Color.RGB which is the correct linear model?

[hlindset]

Nice catch. Should we use Color.RGB which is the correct linear model?

That's what it does now: :scrgb resolves colors with Color.RGB in the :SRGB working space. My bad, forgot to include that in the PR body!