general success

I implemented things as described originally.

the main stumbling block was the arithmatic encoder (ok, I had sort of
beaten together some previously, but they were not exactly completely
working...).
eventually I ended up altering an existing encoder/decoder to work (it is
still kind of a mess, but it is working at least).

status:
lossless compression of lightmap data;
plain uncompressed size: about 10MB;
the various filtering and rle compression: about 400kB;
the various filtering and arithmatic encoding: about 87kB.

I will regard this as a success, and one of the few compression related ones
thus far for me.
most likely for "realistic" images results will be worse, but should be
tolerable (in general for things like textures I am using jpeg, which
results in a libjpeg dependency for me...).

or such...