Lektor thumbnail: set colorspace in imagemagick

People tend to feed lektor any JPG files - also with e.g. Photoshop-CMYK-JPGs.

But JPGs with uncommon colorspaces tend to compress very badly and so on.
For me, it resulted in JPG files wtith a few MB filesize on the website.

I propose to add -colorspace sRGB (it’s the most common in the web) in every imagemagick operation.

[xlotlu]

I personally lean towards the initial suggestion: hardcode it for any thumbnail operation. Given that browsers have to do the conversion anyway, and a thumbnail ready for print doesn’t make sense, I don’t even see a point in making it a default and giving control to the users. (… it may be a good idea for some future-planned operations, like simple conversion, but not for thumbnailing).

The one question that needs answering is if it’s the right / safe thing to do in all cases. A short and inconclusive test shows that imagemagick doesn’t touch grayscale images, despite the -colorspace sRGB, and in all other cases… everything uses sRGB, correct?

P.S.: I’d even go so far to make -strip default, or at least +profile '*' (as strip clears both profiles and comments). I’ve read someplace that browsers assume sRGB if they don’t find an embedded profile, but this should be investigated.

[nixjdm]

Changed in https://github.com/lektor/lektor/pull/694