Add thumbnails to dash

[iBicha]

This is an attempt to close #3435

This allows seeking with a preview in Playlet
Here's what it looks like
It's not perfect, but it's a start

Note: I made sure that this is behind a flag (thumbnails) so that it does not change the behaviour for anyone who did not opt-in

[iBicha]

A little bump to draw the attention

[iBicha]

@unixfox @SamantazFox sorry for the ping, I'm not sure if this PR is in some sort of queue, or forgotten
Thanks

[SamantazFox]

@iBicha yeah, sorry, I haven't been much active this past month (the PR backlog have been filling up :c). I'll try to review and test that soon!

[iBicha]

@iBicha yeah, sorry, I haven't been much active this past month (the PR backlog have been filling up :c). I'll try to review and test that soon!

Thank you, I know finding time for FOSS is hard! Cheers

[iBicha]

@SamantazFox I'll refactor this after #4153 is merged

[iBicha]

Reflecting on this PR a bit, I will be closing this.
I'm trying to add DASH features primarily to support Playlet. I felt that the DASH generation belongs in Invidious, since it is a standard. But the more I dig into it, I discover limitations that are not part of the standard, and that are specific to Roku.
I asked Roku specifically about supporting tiles with different sizes (like in our case with the last page) and their response was in the lines of

...There are all kinds of variations in DASH. Based on needs we could implement those but this needs to be based on a business-related need (like loss or revenue and so on).

So there will always be quirks or limitations, and they will not be prioritized unless they can justify it.

With that reasoning, I'll will be advocating to add things to Invidious that are not standards, or perhaps includes hacks and workarounds, so they fit the Roku OS. That does not sound right, that's why I'll be moving this piece to Playlet so I can cater to these cases (there are many differences between a standard dash player and Roku, including codecs, languages, containers and so on).

Sucks, but it is what it is.

Thank you!