Support JBIG2Decode filter

[gunnsth]

Add support for decoding and encoding with the JBIG2 standard.

See section 7.4.7 JBIG2Decode Filter in the PDF reference (PDF32000_2008):

The JBIG2Decode filter decodes monochrome (1 bit per pixel) image data
that has been encoded using JBIG2 encoding.

• JBIG stands for the Joint Bi-Level Image Experts Group, a group within the ISO that developed the format. JBIG2 is the second version of a standard originally released as JBIG1.
• JBIG2 encoding which provides for both lossy and lossless compression, is only useful monochrome images, not for color images, grayscale images or general data.
• The algorithms are described in ISO/IEC 11544 published standard for the current JBIG2 specification.
• In general JBIG2 provides considerably better compression than the existing CCITT standard.

The optional parameters for JBIG2Decode filter in PDF are:

• JBIG2Globals - a stream containing the JBIG2 global (page 0) segments.

See also Example 1 in the standard which can be used as a testcase.

Implementation

• Makes sense to implement as a package jbig2 that can be included internally in unidoc. Should be licensed with the same license as the unidoc project.
• Start by focusing on decoding, can use the example provided in the PDF reference, and extract some JBIG2Decoded data from PDF files
• Code should follow the unidoc style guide
• Encoding should also be implemented in the package

Notes

I am currently not aware of any golang implementations of JBIG2. However, there are a few open source implementions in other languages that might be a good reference.

• https://github.com/agl/jbig2enc
• https://github.com/apache/pdfbox-jbig2
• https://github.com/Borisvl/JBIG2-Image-Decoder

[gunnsth]

Decoder implemented by #67 #121 and first released in unipdf v3.0.1.

[gunnsth]

Support has been added for generic encoding, see https://unidoc.io/news/jbig2-support-in-golang