Seek to independent IFD sequence in decoding #284
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This serves two purposes: reliably resetting the state of the decoder after kinds of failure; and moving the decoder to a SubIFD chain.
A bit nasty, since we are storing the current Directory within the Image instead of a separate attribute so we can not have one without the other directly. Instead we prioritize with another attribute.
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This adds two capabilities to the encoder:
next_directoryto decode a directory without already decoding its containedimagetags. It only reads a single contiguous range of file data and is thus quite a lot cheaper if only very specific tag values are of interest (or the image is to be skipped).current_directory_as_imageto manually augment the IFD with its contained image.restart_at_{directory,image}to setup a new root directory. This can also be used to iterate SubIFDs in arbitrary depth by combining it withnext_directoryand appropriate tag reads.