Edit > Fade: improve comments
PDI interface: rewrite all write functions to operate on pointers (instead of byte arrays)

A bit nerve-wracking to make this change, as even a single tiny bug
could cause catastrophic program-wide failure, but I *think*
everything's been done carefully.  (Local testing shows no signs of
trouble, at least.)

Despite the risks of passing raw pointers among complicated layers of VB
functions, I think this change worth it, for a single huge advantage:
PDI write time is cut approximately in half, as we cut the number of
array copies required in half (which are by far the limiting factor in
PDI file creation performance, at least among elements under our
control).  Because PDI files are used by the Undo engine as well, this
makes all Undo/Redo creation - and by extension, the general UI of the
program - much snappier.