Add a way to get Video duration

[totalgee]

I see this has been asked before on the forum, but I didn't see any issue entered here. I understand that "Theora video is hard", so maybe this is truly not possible(?). Anyhow, it would be really nice to have -- in particular for the case of video files without audio, because the workaround when they do contain audio is fine (get the audio Source duration).

The closest workaround I found (for video files without audio tracks) was to play the video and then get the tell time when it stops playing, but this is obviously not ideal, since you don't necessarily know if it stopped playing because someone requested a "pause", or because it actually finished... Another workaround I tried was doing a very long seek (like to 10000 seconds, definitely longer than my video), hoping it would stop at the end and then I could ask for the tell position, but it doesn't seem to do that.

Thanks for any solutions or workarounds!