lavf/movenc: Use a dynamic buffer when writing the mfra box

lavf/movenc: Use a dynamic buffer when writing the mfra box

When doing streamed output, with e.g. +dash, if the mfra box ended
up being larger than the AVIOContext write buffer, the (unchecked)
seeking back to update the box size would silently fail and produce
an invalid mfra box.

This is similar to how other boxes are written in fragmented mode.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>