Verify position of first video frame

[ghost]

It is possible for a WebM to pass verification but contain a trimming defect that causes video frame data to be dropped. The effect is a "freezing" of the first frame from the start position to the first valid video frame while the audio plays back successfully.

Example: https://i.fiery.me/H6Nlc.webm

We should video read frame data, or at least retrieve the first video frame, in order to detect these issues. It is possible that we will have false positives depending on the position that we choose to flag this, so this test should be documented as such.