qtdemux: Fix seek adjustment with SNAP_AFTER flag
Fixes #1730 (closed).
Current version, when GST_SEEK_FLAG_SNAP_AFTER
was used, incorrectly adjusted seeks to the latest of keyframes found during the process, instead of the earliest (closest to the desired time) next one. As seen in the linked issue, this was causing problems when one of the streams has more time between key units than the other, resulting in jumps larger than expected.