demux: wav: more robust chunk parsing
From the spec:
"There are no restrictions upon the order of the chunks within a WAVE file,
with the exception that the Format chunk must precede the Data chunk."
The Wav demuxer can now parse chunks in any order. I don't check that the fmt
chunk is before the data one in order to be more resilient to broken samples.
This commit will allow to add new chunks more easily, specially chunks that can
be after the 'data' one. I'm thinking about the ADM support, that need 2
chunks, 'chna' and 'axml' that is after the 'data' chunk.