Prepares extensibility for the eventual case of supporting multi-planar
image layouts that are to be written into a multi-planar buffer as
indicated by the caller in some way (possibly not via read_*_bytes but a
method taking a wrapper around the output buffer instead). Through the
struct we can later add an indicator that can be used by the caller to
decide between methods of fetching the pixel data as appropriate.
This currently avoids buffering the collection, but not buffering them
individually where we still format it into a byte vector.
The main drive here is making the DirectoryEncoder less dependent on the
values. That is, it should be concerned with the Entry representation of
values only. A few decoders expect all values to strictly follow the
image directory and the data layout of such files should be controllable
by the user. This could be achieved by encoding dummy entries which are
later corrected once the actual item is known by seeking back and
writing the directory anew.
By no longer assuming values to be available on finishing a directory we
also prepare an interface that encodes an existing directory.
The previous strategy could not keep track of all cycles such as
reflexive ones at the very start. Additionally, it would fail to be able
to iterate after seeking since it was overly cautious and verified the
next IFD position before allowing the read of a current IFD.
The new implementation is furthermore proof against having multiple
independent chains of IFD which may occur when iterating SubIFDs for
thumbnails. (Note that we do not verify the tree-structure, only a
freedom from cycles).
Stephen Judkins