Change the specification to be less strict in some cases.

In the following three cases we allow more choices
for the compressor, which can potentially lead to
less compressed bits.

  (1) Allow brotli streams where the block counts
      do not count down to exactly zero at the end
      of the meta-block. This makes it possible
      for compressors to sometimes choose a block
      count which can be represented with less bits
      than the exact block count.

  (2) Remove the restriction that prefix code
      descriptions with exactly one non-zero
      length symbol in the code length alphabet
      must have 1 bit depth. This is because
      bit depth 1 requires the most bits to encode.

  (3) Allow any copy length value in the last
      command where the copy part is ignored.
      This makes it possible for a compressor
      to choose a copy length which can be
      represented with the least amount of bits.

In addition to the changes above, this commit also
has a wording clarification in the overview section
where the use of the 'context ID' expression is
changed to be consistent with the rest of the
specification, i.e. that it is a function of the
last two literals or the copy length.

docs/draft-alakuijala-brotli-02.nroff

@@ -372,13 +372,12 @@ called the context map, is encoded in a compact form in the meta-
 block header.
 
 For example, the prefix code to use to decode L2 depends on the
-block type (1), the literal context ID for block type 1 defined
-in the meta-block header,
-and the two uncompressed bytes that were decoded from L0 and L1.
+block type (1), and the literal context ID determined by the two uncompressed
+bytes that were decoded from L0 and L1.
 Similarly, the prefix code to use to decode D0 depends on the block
-type (0), the distance context ID for block type 0, and the copy
-length decoded from IaC0. The prefix code to use to decode IaC3
-depends only on the block type (1).
+type (0), and the distance context ID determined by the copy length decoded
+from IaC0. The prefix code to use to decode IaC3 depends only on the block
+type (1).
 
 In addition to the parts listed above (prefix code for insert-
 and-copy lengths, literals, distances, block types and block counts
@@ -735,9 +734,8 @@ follows:
       length. In this case, that symbol results in no bits
       being emitted by the compressor, and no bits consumed by
       the decompressor. That single symbol is immediately
-      returned when this code is decoded. (If the ignored non-
-      zero length is not 1, then the stream should be rejected
-      as invalid.)  An example of where this occurs is if the
+      returned when this code is decoded.
+      An example of where this occurs is if the
       entire code to be represented has symbols of length 8.
       E.g. a literal code that represents all literal values
       with equal probability. In this case the single symbol
@@ -969,9 +967,9 @@ type of the first block switch command is not encoded in
 the compressed data. Instead the block count for each category
 that has more than one type is encoded in the meta-block header.
 
-The block counts for all three categories should count down to exactly
-zero at the end of the meta-block. If any do not, then the stream
-should be rejected as invalid.
+Since the end of the meta-block is detected by the number of uncompressed
+bytes produced, the block counts for any of the three categories need not
+count down to exactly zero at the end of the meta-block.
 
 The number of different block types in each block category, denoted
 by NBLTYPESL, NBLTYPESI, and NBLTYPESD for literals, insert-and-copy
@@ -1598,9 +1596,8 @@ The decoding algorithm that produces the uncompressed data is as follows:
             read literal using HTREEL[CMAPL[64 * BTYPE_L + CIDL]]
             write literal to uncompressed stream
          if number of uncompressed bytes produced in the loop for
-            this meta-block is MLEN, then break from loop (if the
-            discarded copy length is not 4, then reject the stream as
-            invalid)
+            this meta-block is MLEN, then break from loop (in this
+            case the copy length is ignored and can have any value)
          if distance code is implicit zero from insert-and-copy code
             set backward distance to the last distance
          else