fix: computeLivePosition minimum value of media.currentTime

[jony89]

This PR will...

make computeLivePosition to have a minimum value of media.currentTime. and will prevent seeking in case liveSyncPosition <= media.currentTime

Why is this Pull Request needed?

hls.js seeks backwards during live streaming #2494

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

we could just add the liveSyncPosition > media.currentTime condition without touching computeLivePosition function but I think this is more correct that way.

Resolves issues:

#2494

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

[jony89]

@robwalch done.

As I wrote

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

we could just add the liveSyncPosition > media.currentTime condition without touching computeLivePosition function but I think this is more correct that way.

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I really think that computeLivePosition should not return value that is less than the current time as it literally says live. but I've made the requested changes.

[robwalch]

I really think that computeLivePosition should not return value that is less than the current time as it literally says live. but I've made the requested changes.

jony89 thanks for making the changes.

computeLivePosition should return a value based on the ideal position. That include allowing enough buffer for the stream to not underbuffer. It doesn't need to know anything about the where the video is. Limiting it to currentTime prevents it from return a safe value, and adds code that we're not using since you're already capping where we can seek to.