Fix utf-8 encode fragment text issue

[dsparacio]

POST PR #1057 fragmented text broke based on comments below.

It is indeed the access to UTF8.decode in TextSourceBuffer.js which has been lost.

Replacing that decoder with the lines:

                let dataView = new DataView(bytes, sampleList[i].offset, sampleList[i].size);
                ccContent = new TextDecoder('utf-8').decode(dataView);

makes fragmented subtitling work again.
Using a data view is also more efficient than the current approach, but the approach above only works on brewers with TextDecoder.
The codem-isoboxer that we include has a fallback, but a poor one. I've made a PR to it with a better fallback (essentially the one from base64) together with new boxes for emsg. Maybe we can use it..

[dsparacio]

Fixed by PR #1105 thanks @TobbeMobiTV !!