The problem was that we were stripping color profiles from thumbnails,
but we weren't setting `export_profile: "srgb"` to convert images to
sRGB first. This resulted in wrong colors for images with non-sRGB color
profiles, such as Adobe RGB.
The fix is to convert images to sRGB when possible, while leaving CMYK
and greyscale images alone. We leave CMYK images alone because we can't
convert CMYK to sRGB without losing color. We leave greyscale images
alone if they don't have a color profile, that way they stay as
one-channel greyscale (or two-channel greyscale, in case of alpha)
instead of being converted to three-channel sRGB. However, if a
greyscale image has a color profile, then we have to convert to sRGB,
otherwise the colors would be wrong when we strip the profile.
We also have to set the import profile, otherwise images with broken
embedded color profiles won't have a fallback profile and may get
incorrect colors. In this case we also have to be careful, because we
can't specify an sRGB fallback for greyscale or CMYK images.
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