Enabled for quality >= 4, and if there are no obvious
UTF8 violations detected.
For each block, we gather two separate histograms, one
for continuation bytes and one for ASCII or lead bytes.
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.
The new mode can be used by setting the greedy_block_split
field of BrotliParams to true.
This commit moves all the meta-block processing code
into its own library and moves the meta-block encoding
code to brotli_bit_stream.cc from encode.cc
