The run encoding scheme byteRun1 is best described by pseudo code for the
decoder Unpacker (called UnPackBits in the Macintosh(tm) toolbox):

  UnPacker:
    LOOP until produced the desired number of bytes
        Read the next source byte into n
        SELECT n FROM
            [0..127]   => copy the next n+1 bytes literally
            [-1..-127] => replicate the next byte -n+1 times
            -128       => no operation
            ENDCASE;
        ENDLOOP;

In the inverse routine Packer, it's best to encode a 2 byte repeat run as
a replicate run except when preceded and followed by a literal run, in
which case it's best to merge the three into one literal run.  Always
encode 3 byte repeats as replicate runs.

Remember that each row of each scan line of a raster is separately packed.