Fix parsing multiple user data unregistered SEI

[ioppermann]

This PR will...

allow to parse more than the first user data unregistered SEI per video frame.

Why is this Pull Request needed?

It may be required by a player to have access to all user data unregistered SEI in order to control playback or to display additional information to the viewer.

Are there any points in the code the reviewer needs to double check?

I was wondering why only the first SEI is parsed (endOfCaptions set to true after encountering a supported payloadType) and couldn't find any documentation why this is the case. In the case of CC (payloadType 4) I could imagine that it might be problematic if there's more than one (looking at controller/timeline-controller.ts). But for unregistered user data (payloadType 5) it shouldn't cause any issues.

I also noticed that payloadtype 5 uses userDataBytes in the samples (instead of bytes as defined in the UserdataSample interface). If it would use bytes it would probably screw up the onFragParsingUserdata method in controller/timeline-controller.ts. This PR doesn't mess with this.

Resolves issues:

none

Checklist

• changes have been done against master branch, and PR does not conflict
• new unit / functional tests have been added (whenever applicable)
• API or design changes are documented in API.md