...from "bytes" to "size_in_bytes".
I thought at first that `bytes` meant the variable contained
bytes that the decoder would read from.
Also, this variable is called `sz` (for `size`) in the spec.
No behavior change.
This doesn't appear to have had a measurable impact on performance,
and behavior is the same.
With the tiles using independent BooleanDecoders with their own
backing BitStreams, we're even one step closer to threaded tiles!
