Psd-tools3

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- Highly experimental: basic layer merging is implemented
(e.g. it is now possible to export layer group to a PIL image);
- ``Layer.visible`` no longer takes group visibility in account;
- ``Layer.visible_global`` is the old ``Layer.visible``;
- ``psd_tools.user_api.combined_bbox`` made public;
- ``Layer.width`` and ``Layer.height`` are removed (use ``layer.bbox.width``
and ``layer.bbox.height`` instead);
- ``pil_support.composite_image_to_PIL`` is renamed to ``pil_support.extract_composite_image`` and
``pil_support.layer_to_PIL`` is renamed to ``pil_support.extract_layer_image``
in order to have the same API for ``pil_support`` and ``pymaging_support``.

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- ``psd.composite_image()`` is renamed to ``psd.as_PIL()``;
- Pymaging support: ``psd.as_pymaging()`` and ``layer.as_pymaging()`` methods.

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- Support for zip and zip-with-prediction compression methods is added;
- support for 16/32bit layers is added;
- optional Cython extension for faster zip-with-prediction decompression;
- other speed improvements.

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- Initial support for 16bit and 32bit PSD files: ``psd-tools`` v0.2 can
read composite (merged) images for such files and extract information
(names, dimensions, hierarchy, etc.) about layers and groups of 16/32bit PSD;
extracting image data for distinct layers in 16/32bit PSD files is not
suported yet;
- better ``Layer.__repr__``;
- ``bbox`` property for ``Group``.

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Packaging is fixed in this release.

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- Better support for 32bit images (still incomplete);
- reader is able to handle "global" tagged layer info blocks that
was previously discarded.