Optional Content Groups / Layers manipulation

[johnwhitington]

For the record, in master:

• -list-ocgs-json implemented
• -replace-ocgs-json implemented for roundtripping
• Verify all non-JSON OCG functionality currently in Cpdf is effectively subsumed by (1) and (2).
• Merge should merge OCGs?
• Extend -stamp-on, -stamp-under, -combine-pages to allow layers to be created, one for each file. What happens if layers already exist? Tree or flat? Optional.
• Allow splitting to file into its composite layers all at once, or one at a time. (By genuinely removing the data not merely by toggling which layers are shown? Maybe only eventually, depending on how hard this is.)
• Add functions to create OCGs with -draw
• Add -create-ocg to make one layer from whole file (tree or singular if non-existant)
• Add -flatten-ocg to remove all OCGs from a file
• Add -delete-ocg to remove a single layer.
• Add -strip-ocg to remove all optional content, leaving only that which is not optional
• Replace all existing functionality in cpdfocg.ml with work through the new, complete, OCG type.

(This work is pausing now, for other things. But we will get back to it)