Add the option to provide a specific background colour to trim

[mart-jansink]

I've got a use case to trim just a specific background colour of an image. Mostly to detect letter and pillar boxes in images by trimming off any black areas. This pull request adds the option to do so, by providing this colour as a string or object, which will be parsed by the color module.

For me, it'd be more typical to provide a specific colour and use the default threshold, so I made the former the first optional argument. However, the Javascript side will handle the case where a threshold is provided as the first argument to retain backwards compatibility. I'm not sure if I made it fully clear in the docs that both arguments are optional, maybe you can improve my phrasing if you agree?

The output of the color module is always 8-bit, so it's scaled accordingly if the image is 16-bit. I've noticed that the threshold option is used as an absolute value in find_trim instead of being scaled relatively to the interpretation, therefore that's now multiplied as well.

Instead of .flatten()ing the background color if the image HasAlpha(), the alpha value is now simply discarded. I believe that this should give the same result because of how find_trim uses the same background color when it flattens the image. At least all existing unit tests still pass.

Finally, I've based this pull request on the eagle branch because e0d3c6e and cbf741c affect the same part of the code. And I also encountered some failing tests when running the main branche on my M1 MacBook Pro.

[lovell]

Thank you very much for the PR, this is a welcome addition, I've left a couple of questions/comments inline.

[mart-jansink]

Could you please have another look? I've modified the pull request according to your suggestions.

Also, it now allows the threshold to be zero to trim just the pixels with the exact same color. I couldn't think of any reason why this shouldn't work. Of course I'll revert it if there was one, but https://github.com/libvips/libvips/blob/f4a3c7eb5478ee77bd206353308eab3b02db923a/libvips/arithmetic/find_trim.c#L193 seems to be fine with anything between 0 and INFINITY inclusive.

[coveralls]

Coverage remained the same at 100.0% when pulling 526b7e2 on mart-jansink:trim-specific-colour into 3a44748 on lovell:eagle.

[lovell]

Dankjewel Mart!