#844 RichPNG metadata within PDF files

[Jeremy Faden]

I just ran across a script (https://github.com/autoplot/dev/blob/master/demos/2019/20191021/itextMeta.jy) which demonstrates that grep-able metadata can be embedded within a PDF. I forgot that this was possible, and richPNG metadata (with axis titles, positions, and ranges) should be embedded. Then this should also be supported within the PNG Walk Tool, which can also browse PDF files.