Remux a fragmented MP4 blob into a flat (non-fragmented) MP4.
If the input is already a flat MP4 or remuxing fails, returns the
original blob unchanged.
Parameters
blob: Blob
The fragmented MP4 blob from MediaRecorder
OptionalmimeType: string
The MIME type for the output blob (default: blob.type or 'video/mp4')
Returns Promise<Blob>
A flat MP4 blob suitable for cross-browser playback
Remux a fragmented MP4 blob into a flat (non-fragmented) MP4. If the input is already a flat MP4 or remuxing fails, returns the original blob unchanged.