webp-container-spec: make padding byte=0 a MUST
this is consistent with the RIFF specification.
based on comments from:
@@ -128,9 +128,8 @@ Chunk Size: 32 bits (_uint32_)
Chunk Payload: _Chunk Size_ bytes
-: The data payload. If _Chunk Size_ is odd, a single padding byte -- that
- SHOULD be `0` to conform with RIFF -- is added. Applications MAY use
- another value, but readers may fail to parse the file.
+: The data payload. If _Chunk Size_ is odd, a single padding byte -- that MUST
+ be `0` to conform with RIFF -- is added.
**Note:** RIFF has a convention that all-uppercase chunk FourCCs are standard
chunks that apply to any RIFF file format, while FourCCs specific to a file
