Plone.scale

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- PIL thumbnail does not work for magnifying images (when scaling up).
Use resize instead. [sureshvv]

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- When a scale is outdated, discard all image scales that are more
than a day older than the context.
Refs https://dev.plone.org/ticket/13791
[maurits]

- Make sure deleting items or clearing a complete storage works.
Deleting one item would often delete a linked second item, which
made it hard to remove several items at once.
[maurits]

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- Discard old image scales if item was modified.
Refs https://dev.plone.org/ticket/13791
[gforcada]

- Generate Progressive JPEG.
[kroman0]

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- Added a marker interface for scaled image quality.
Refs http://dev.plone.org/plone/ticket/13337
[khink]

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- Cropped images are now centralised vertically as well as horizontally [mattss]

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- Add MANIFEST.in.
[WouterVH]

- Break up `scaleImage`, so that its scaling-related parts can be applied
to instances of `PIL.Image` for further processing.
[witsch]