Duplicate scales are generated when the image hasn't been modified about plone.scale

[davisagli]

It's even worse than I thought: actually a new copy of the large scale blob is being created each time I view the image, even if I have not edited it.

[davisagli]

I think the problem is that when the real scale is generated using plone.scale's AnnotationStorage.get_or_generate, it is stored using a different uid than where the placeholder scale info was stored. This happens because plone.namedfile.scaling.ImageScaling looks up IImageScaleStorage without providing a way to get the field's modified time (which would be difficult, because it doesn't necessarily know what field it is).

Fixing it so that the actual field modification time is available looks complicated, but I have a simpler fix in mind: make get_or_generate pass the existing uid to generate_scale, so that generate_scale can use it and skip re-calculating the uid.