Embedded audio gives base64 decode error.

[nigelmegitt]

When base64 decoding the audio in:

• ttml2-prstn-audio-embedding-chunked.xml and
• ttml2-prstn-audio-embedding-sourced.xml

there's an invalid character error (input to base64 formed by creating a text file from the source data):

$ base64 -D -i chunked-source-base64.txt -o chunked-source-audio.wav
Invalid character in input stream.

I will propose a pull request that replaces the base64 encoded data with audio data that does not produce this error.

[skynavga]

Actually, it was my intention to permit normal XML whitespace characters in base64 data, which is permitted [1], and which would be ignored by decoders [2]. However, I see that RFC4648 wants the referring spec (in our case TTML2) to explicitly sanction such usage. I will add an issue (for resolution in 2e) to explicitly permit these characters within base64 data.

[1] https://tools.ietf.org/html/rfc4648#section-3.1
[2] https://tools.ietf.org/html/rfc4648#section-3.3