Refactored PdfObjectString into a struct with bool flag for hex. Fixed any code broken by the change.
Unexported non-essential functions for crypto (not used by model). Can unexport more later or refactor to internal package.
Support avg, paeth predictors for decoding. Special error flag for
unsupported error parameters. Support specifying encoder when creating
new image objects (inline,xobject).
As the package was growing large, there is a need to clean up. Testing
to refactor into a core (basic pdf parsing and primitive objects) and a
higher level model package.
