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[PATCH] Detect Keyframes in WebM streams

The existing WebM plugin will mark the start of any cluster as a sync point. This causes problems for web browsers, which tend to strictly demand that video streams are started on a keyframe.

The WebM specs call for a keyframe being the first video frame in a cluster if the cluster contains a keyframe, but they do not require clusters to contain any keyframes at all.

This patchset:

  1. implements some EBML parsing functions, to identify clusters accurately instead of assuming any occurrence of the cluster magic number is a cluster header.
  2. detects if a WebM stream contains a track of type video.
  3. examines the SimpleBlocks of each cluster to determine if the first video track block is marked as a keyframe.

If the first video block is a keyframe, the cluster is marked as a sync point. If the first video block is NOT a keyframe, the cluster is not marked as a sync point.

If a determination cannot be made in the first 4K of the cluster, it's optimistically marked as a sync point, since it's most likely an audio-only stream.