pngexif: Import pngexifinfo as an externally-contributed project

We used this experimental project in the development of the PNG-EXIF
("eXIf") specification, back in 2017. The project evolved together
with the draft specification, which was finalized on 2017-Jun-15 and
approved by the PNG Group on 2017-Jul-13.

The EXIF specification, outside of the scope of PNG and libpng, is
quite complex. The libpng implementation cannot grow too much beyond
performing basic integrity checks on top of serialization. In order
to create and manipulate PNG-EXIF image files, the use of external
libraries and tools such as ExifTool is necessary.

Now, with the addition of contrib/pngexif to the libpng repository,
offline tasks like metadata inspection and linting can be performed
without importing external dependencies.

contrib/pngexif/.editorconfig

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+# https://editorconfig.org
+
+root = true
+
+[*]
+charset = utf-8
+indent_style = space
+insert_final_newline = true
+max_doc_length = 79
+max_line_length = 79
+trim_trailing_whitespace = true

contrib/pngexif/.gitignore

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+__pycache__
+*.py[co]
+*$py.class

contrib/pngexif/.pylintrc

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+[COMPATIBILITY]
+disable=consider-using-f-string
+
+[COMPLEXITY]
+disable=too-many-branches,too-many-instance-attributes
+
+[STYLE]
+disable=consider-using-in

contrib/pngexif/LICENSE_MIT.txt

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+MIT License
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

contrib/pngexif/README.md

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+pngexifinfo
+===========
+
+Show the EXIF information embedded in a PNG file.
+
+
+Sample usage
+------------
+
+Show the EXIF info inside a PNG file:
+
+    pngexifinfo /path/to/file.png
+
+Show the EXIF info inside a raw `.exif` file, using base 16 for the EXIF tags:
+
+    pngexifinfo --hex /path/to/file.exif
+
+Show the help text:
+
+    pngexifinfo --help

contrib/pngexif/bytepack.py

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+#!/usr/bin/env python
+
+"""
+Byte packing and unpacking utilities.
+
+Copyright (C) 2017-2020 Cosmin Truta.
+
+Use, modification and distribution are subject to the MIT License.
+Please see the accompanying file LICENSE_MIT.txt
+"""
+
+from __future__ import absolute_import, division, print_function
+
+import struct
+
+
+def unpack_uint32be(buffer, offset=0):
+    """Unpack an unsigned int from its 32-bit big-endian representation."""
+    return struct.unpack(">I", buffer[offset:offset + 4])[0]
+
+
+def unpack_uint32le(buffer, offset=0):
+    """Unpack an unsigned int from its 32-bit little-endian representation."""
+    return struct.unpack("<I", buffer[offset:offset + 4])[0]
+
+
+def unpack_uint16be(buffer, offset=0):
+    """Unpack an unsigned int from its 16-bit big-endian representation."""
+    return struct.unpack(">H", buffer[offset:offset + 2])[0]
+
+
+def unpack_uint16le(buffer, offset=0):
+    """Unpack an unsigned int from its 16-bit little-endian representation."""
+    return struct.unpack("<H", buffer[offset:offset + 2])[0]
+
+
+def unpack_uint8(buffer, offset=0):
+    """Unpack an unsigned int from its 8-bit representation."""
+    return struct.unpack("B", buffer[offset:offset + 1])[0]
+
+
+if __name__ == "__main__":
+    # For testing only.
+    assert unpack_uint32be(b"ABCDEF", 1) == 0x42434445
+    assert unpack_uint32le(b"ABCDEF", 1) == 0x45444342
+    assert unpack_uint16be(b"ABCDEF", 1) == 0x4243
+    assert unpack_uint16le(b"ABCDEF", 1) == 0x4342
+    assert unpack_uint8(b"ABCDEF", 1) == 0x42