STAD           *.PAC (high resolution only)

4 bytes        "pM86" (vertically packed) or "pM85" (horizontally packed)
1 byte         id byte
1 byte         pack byte (most frequently occuring byte in bitmap)
1 byte         "special" byte
7 bytes        total for header

? bytes        data

The data is encoded as follows.  For each byte x in the data section:

   x = id byte             Read one more byte, n.  Use pack byte
                           n + 1 times.
   x = "special" byte      Read two more bytes, d, and n (in order).
                           Use byte d n times.
   otherwise               Use byte x literally.