Initial implementation of the main process() function.

[veluca93]

Summary of the implementation choices:

• There are two different parsers, one for boxes and one for the codestream.
• The codestream parser pulls data from the box parser
• The codestream parser tries to minimize memory usage by putting data into buffers of the size of single sections.
• Data might be not read if it turns out not to be needed.

Still TODO:

• Integrating this code with jxl_cli and testing it
• Support output in different pixel formats
• Support output with different color profiles
• Implement flush_pixels()
• Generate BasicInfo and Frame information
• Integrate writing output in the render pipeline.