change thumbnail normalization

[jrochkind]

john cupitt of vips suggests:

Secondly, P3 colourspace is becoming very widely used. This is significantly bigger than sRGB, and P3 images converted to sRGB will either be drastically clipped or desaturated. Either way, your users are likely to complain, especially the photographers.

Rather than forcing everything to srgb, I think best practice now is to strip all metadata except the icc profile.

https://github.com/janko/image_processing/issues/34#issuecomment-923854366