Background video playback support

[Titaniumtown]

Currently the Background button in grayjay has no effect on currently playing Nebula content. The video briefly disappears, then comes back within 0.1s.

[Zvonimir-FUTO]

Affects all plugins that use HLS.

[kaidelorenzo]

This also applies to any video that doesn't contain a separately specified audio source. For platforms that only provide a single video file that contains audio and video in the same file it likely doesn't matter that much because bandwidth usage is going to be the same. However for DASH and HLS we should make sure that the video part doesn't stream when playing in the background

[kaidelorenzo]

Should be fixed by this development that will hopefully make it into the next Grayjay release

https://gitlab.futo.org/videostreaming/grayjay/-/merge_requests/80

[VoxelPrismatic]

The 'background' button is designed to not stream video. Like Twitch's "audio only" button. Rumble & Nebula doesn't support this.