Hello FFmpeg developers,

This patch fixes the calculation of bit_rate for fragmented MP4 files,
ensuring
that the reported bit_rate covers the entire fragmented file instead of only
the first fragment.

This addresses GitHub issue: https://github.com/FFmpeg/FFmpeg/issues/20450

Changes include modifications in:
- libavformat/mov.c
- libavformat/isom.h

Tested using ffprobe on sample fragmented MP4 files (12.mp4 and 21.mp4), and
the bit_rate is now calculated correctly over the whole file.

Patch attached.

Best regards,
 Nariman Sayed