pdPackage files can now be read and parsed

In theory, this class should now be capable of reading/writing valid
pdPackage files without trouble.  (Though I'm reminded of the Simpsons
episode where Homer says: "in theory, communism works, Marge... IN
THEORY.")

I'm now ready to start writing an interface that takes XML and binary
data from a pdImage object (and all its child layers), preps the data,
then passes it to a pdPackage instance so it can be written to file.

Fingers crossed that it all works on the first try!  /s
PhotoDemon can now read and write layered images (custom PDI format)

YIPPEE-KI-YAY!  I honestly didn't expect this to work so easily, which
makes me wonder if I have missed something obvious...

AFAICT, the new pdPackager class works brilliantly.  PDI files can be
now written and read in both compressed and uncompressed format, and the
resulting files are significantly smaller than an identical image in XCF
format (GIMP's native format).  This is without any specialized
compression business, just raw zLib streams!  Based on this, I think my
strategy of writing all headers as compressed XML is a valid one,
without too much penalty to file size.

Checksums work wonderfully, and saving/loading is remarkably fast,
especially considering that I have yet to optimize some sloppy stuff
like ReDim Preserve statements that could be eliminated with better
handling.

I will be doing a ton of stress testing in the weeks to come, but so
far, I am greatly encouraged by this early success.