Plone-scale

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New:

- Assume a width or height of zero is semantically the same as None already was:
Use the other dimension to scale, calculate the missing one.
[jensens, thet]

- Scaled GIFs are converted to RGBA PNG images instead of converting them to JPEG.
[thet, jensens]

Fixes:

- Don't scale images up for direction "down".
[thet]

- Major housekeeping, code refactored in order to reduce complexicty.
[jensens]

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New:

- Use an adapter to lookup the actual factory for scaling.
Deprecated passing the factory as named parameter along,
because this had not enough flexibility:
If addons want to provide alterative methods to scale (i.e. cropping),
now a specific adapter can perform the work.
[jensens]

Fixes:

- Minor housekeeping.
[jensens]

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Fixes:

- Fix KeyError in storage.AnnotationStorage._cleanup when attempting
to delete the storage for the same key twice.
[fulv]

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New:

- Resolve conflicts raised when accessing multiple scales concurrently.
[gotcha]

- Refactored scale storage.
[gotcha]

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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- When a scale is outdated, discard all image scales that are more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]