Updates to Brotli compression format, decoder and encoder

This commit contains a batch of changes that were made to the Brotli
compression algorithm in the last month. Most important changes:

   * Format change: don't push distances representing static dictionary words to the distance cache.
   * Fix decoder invalid memory access bug caused by building a non-complete Huffman tree.
   * Add a mode parameter to the encoder interface.
   * Use different hashers for text and font mode.
   * Add a heuristics to the hasher for skipping non-compressible data.
   * Exhaustive search of static dictionary during backward reference search.

brotli/enc/command.h

@@ -24,9 +24,13 @@ namespace brotli {
 // Command holds a sequence of literals and a backward reference copy.
 class Command {
  public:
+  // distance_code_ is initialized to 17 because it refers to the distance
+  // code of a backward distance of 1, this way the last insert-only command
+  // won't use the last-distance short code, and accordingly distance_prefix_ is
+  // set to 16
   Command() : insert_length_(0), copy_length_(0), copy_length_code_(0),
-              copy_distance_(0), distance_code_(0),
-              distance_prefix_(0), command_prefix_(0),
+              copy_distance_(0), distance_code_(17),
+              distance_prefix_(16), command_prefix_(0),
               distance_extra_bits_(0), distance_extra_bits_value_(0) {}
 
   uint32_t insert_length_;