Use CUES element for seeking if available


Our current seeking algorithm assumes the movie has many keyframes, preferably at least one per cluster. However, some clips only have a single keyframe (at the beginning of the file, in the first cluster), but this breaks seeking, which is currently based on searching through clusters to look for a keyframe.
If a CUES element is present, try to use that first to find the nearest keyframe. Revert to cluster-based seeking only if the CUES element is not present, or it doesn't list any keyframes.