Followup: I've now implemented this and I determined it doesn't take enough longer to have a noticeable impact. It so happens mango has an sRGB to linear function that is much faster than using Pixman to do the conversion so that's what I used. I kept it 32-bit all the way through which will introduce some colour banding but it's not really noticable with the photorealistic images being resized. So I expect this will be ready for whenever I release my next version