Read stream dimensions from stream packets

[luludotdev]

Closes #25

Caveats: First metadata report will use the fallback (wrong) values since keyframe data is likely not ready in time.
Is also worth in future refactoring PreviewGenerators to a more generic interface for video decoding.

[haydenmc]

Hi @lolPants , sorry for the delay on this! Glimesh.tv just launched and we've been scrambling to introduce stability fixes. I promise this change has not been forgotten!

[luludotdev]

Rebased onto latest master, I'm intending to do a bit of cleanup and refactor PreviewGenerator to VideoDecoder. Mostly just internal renames though.

[luludotdev]

Rebased again to resolve merge conflicts.

[luludotdev]

Any updates on this, it's been over a month now.

[haydenmc]

I think this looks good - I have two concerns, but neither of them are things I'd consider blocking:

1.) I don't think we need to decode an entire keyframe to get the width and height of the video - the SPS NAL should contain the width and height, so we could probably just pull that out without having to feed any data through libav at all.
2.) With this change the metadata report thread will now decode each keyframe twice - first to get the resolution, and then again to generate the preview image.

I don't think these are concerning enough to block the merge, so I'm just opening an issue to track here so we make improvements going forward: #120