Psd-tools2

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- fix descriptor decoding errors;
- minor bugfixes.

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- fix UnicodeError in exif;
- workaround for irregular descriptor name;
- add undocumented `extn` tagged block decoding;
- move duplicated icc module to subpackage;
- support PIL rendering with extra alpha channels.

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- update documentation;
- fix PEP8 compliance;
- rename merge_layers to compose.

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- fix white background issue in `as_PIL()`.

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- add quality testing;
- fix disabled mask.

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- fix `has_mask()` condition;
- add mask composition in `merge_layers()`;
- fix mask display.