This mode stores the actual new frame long-words which are different,
along with the offset in the bitmap.  The exact format is shown and
discussed in section 2 below. Each plane is handled separately, with no
data being saved if no changes take place in a given plane.  Strings of 2
or more long-words in a row which change can be run together so offsets do
not have to be saved for each one.

Constructing this data chunk usually consists of having a buffer to hold
the data, and calculating the data as one compares the new frame,
long-word by long-word, with two frames back.

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