Test position of first video frame

[ghost]

If a webm is trimmed or stream copied, it is possible that a video frame will not be defined until the first keyframe is reached and the resulting playback displays the first keyframe until its position is reached in playback.

Example: https://files.catbox.moe/j6q0g1.webm

ffprobe command used in initial scan:

ffprobe -show_frames -select_streams v -read_intervals %+#1 -print_format json input.webm > frames.json

False positives are a possibility here for sequences with black frames at the start of the cut. We should pick a best estimate reference position and warn the user if the first position is past this post.

Suggestion: 0.250s.