Psd-tools

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- fix `has_pixel()` for partial channels;
- support color overlay in `compose()`.

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- fix mask rendering in compose (Thanks andrey-hider and nkato).

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- unicode bugfixes.

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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.