Psd-tools

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- fix broken `psd_tools.composer.vector` module in 1.8.17;
- experimental support for color noise gradient;
- bugfix for clip masks;
- bugfix for CMYK composing.

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- move `psd_tools.api.composer` module to `psd_tools.composer` package;
- support 19 blending modes in composer;
- support fill opacity;
- fix image size when composing with masks;
- rename `TaggedBlockID` to `Tag`;
- rename `ImageResourceID` to `Resource`;
- add `bytes` mixin to `Enum` constants;
- replace `Enum` keys with raw values in `psd_tools.psd.base.Dict` classes.

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- fix broken group compose in 1.8.15;
- fix missing pattern / gradient composition in vector stroke content.

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- coding style fix;
- fix `compose()` bbox option.

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- add dependency to aggdraw;
- support bezier curves in vector masks;
- support path operations;
- fix `compose(force=True)` behavior;
- fix default background color in composer;
- improve pattern overlay parameters support;
- fix gradient map generation for a single stop.

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- fix engine_data unknown tag format;
- fix compose for extra alpha channels;
- workaround for pillow 6.0.0 bug.