WIP: S2++

If the first bytes of a block is `0x40, 0x00` (repeat, length 4), this indicates that all [Copy with 4-byte offset (11)](https://github.com/google/snappy/blob/main/format_description.txt#L106) are all 3 bytes instead for the remainder of the block.

There can be no literals before this tag and no repeats before a match as specified above.
This will only trigger on this exact tag.

> These are like the copies with 2-byte offsets (see previous subsection),
> except that the offset is stored as a 24-bit integer instead of a
> 16-bit integer (and thus will occupy three bytes).

When in this mode the maximum backreference offset is 16777215.

This *cannot* be combined with dictionaries.

s2/README.md

@@ -1022,6 +1022,7 @@ See [using indexes](https://github.com/klauspost/compress/tree/master/s2#using-i
 * Frame [Stream identifier](https://github.com/google/snappy/blob/master/framing_format.txt#L68) changed from `sNaPpY` to `S2sTwO`.
 * [Framed compressed blocks](https://github.com/google/snappy/blob/master/format_description.txt) can be up to 4MB (up from 64KB).
 * Compressed blocks can have an offset of `0`, which indicates to repeat the last seen offset.
+* If the first bytes of a block is `0x80, 0x00, 0x00` (copy, 2 byte offset = 0), this indicates that all [Copy with 4-byte offset (11)](https://github.com/google/snappy/blob/main/format_description.txt#L106) are all 3 bytes instead for the remainder of the block.
 
 Repeat offsets must be encoded as a [2.2.1. Copy with 1-byte offset (01)](https://github.com/google/snappy/blob/master/format_description.txt#L89), where the offset is 0.
 
@@ -1047,6 +1048,19 @@ The first copy of a block cannot be a repeat offset and the offset is reset on e
 
 Default streaming block size is 1MB.
 
+## 3 Byte Offsets
+
+If the first bytes of a block is `0x80, 0x00, 0x00` (copy, 2 byte offset = 0), this indicates that all [Copy with 4-byte offset (11)](https://github.com/google/snappy/blob/main/format_description.txt#L106) are all 3 bytes instead for the remainder of the block.
+
+There can be no literals before this tag and no repeats before a match as specified above.
+This will only trigger on this exact tag.
+
+> These are like the copies with 2-byte offsets (see previous subsection),
+> except that the offset is stored as a 24-bit integer instead of a
+> 16-bit integer (and thus will occupy three bytes).
+
+When in this mode the maximum backreference offset is 16777215.
+
 # Dictionary Encoding
 
 Adding dictionaries allow providing a custom dictionary that will serve as lookup in the beginning of blocks.