implement "always available" streams (#2214)

[aler9]

Fixes #2214

This patch allows streams to be always available to readers, generating continuous streams regardless of whether the publisher is online or not. This works by seamlessly merging together "online" streams with an "offline" video, without decoding or re-encoding anything (as always), acting on already-encoded packets. When a publisher is offline, a blank video is returned:

The feature can be enabled by setting the alwaysAvailable property on specific paths. Track list is also mandatory:

paths:
  stream:
    alwaysAvailable: true
    alwaysAvailableTracks:
    - codec: H264

TODO:

• support multitrack streams
• support audio tracks
• allow to specify an alternative offline video
• add documentation
• add tests

[roger-]

Awesome! Can this be extended to support an arbitrary "offline" image? Having the ability to dynamically set the image would be great too.

[aler9]

@roger- absolutely, i'll add some settings like

alwaysAvailableOfflineVideo: myvideo.mp4

[roger-]

Would it catch a change in myvideo.mp4 and reload?

I'd like to be able to repeat the last frame until input stream resumed, and I can hack together something if myvideo.mp4 isn't cached. FYI my use case is turning an intermittent (battery) camera stream into a continuous one.

[jmduerk]

When using this feature, will the paths/list API always show active for a stream with this configured, or will API return true online/offline status?