Disable all slow features for quality <= 9 (literal cost modeling,
dictionary, context modeling, advanced block splitting).
Change vector<Command> arguments of internal functions
to Command* and size_t.
Add a version of the brotli encoder that compresses
each meta-block independently, only using the
original input data from previous meta-blocks
and nothing from the compressor state.
This is a proof-of-concept to show that the
current format is flexible enough to support
parallel multi-threaded compression.
