Add to the frame info a data_length to pair with data_offset.
Fix issue writing final subblock of length 256.
There is special handling when we have finished and the last subblock was
already completed and a new one not yet started.  This fixes a bug where the
logic was a bit inverted and we wrote out two subblock null terminators.

Bug: https://github.com/deanm/omggif/issues/5