Reproduce partially transparent affine backgrounds exactly

[hlindset]

A partially transparent :background in affine/3, rotate/3, translate/4 and shear/4 came back distorted because libvips injects it raw into a premultiplied space, so the output unpremultiply scaled the color bands by max_alpha / alpha:

Image.affine(image, matrix, background: [10, 20, 30, 40])
# fill came out [63, 127, 191, 40]

Now the image and background are premultiplied first and the affine runs with premultiplied: true.

Note: Since vips_rotate() doesn't expose the premultiplied option, rotate/3 sends alpha images through affine/3instead. vips_rotate() is a wrapper over vips_affine() under the hood, so this is lossless.

[kipcole9]

I knew that I was handling pre-multiplied badly - you fixing this area is really fabulous, thank you.

[hlindset]

I knew that I was handling pre-multiplied badly - you fixing this area is really fabulous, thank you.

Ah, I think the pre-existing premultiply/unpremultiply in resize is fine. This one was actually my doing. A quirk in when libvips inserts the background fill (after premultiply, before unpremultiply) which ended up warping the color bands if the background had alpha.