Draft: mux/mp4: add GIMI variant

GIMI = GEOINT Imagery Media for ISR Profile of ISOBMFF & HEIF

This is being standardised as the MPEG-4 style replacement for MPEG-2 Transport Stream + KLV.

Key features (for video aspects):

• Restricted range of supported formats (requires ISO/IEC 23001-17 work)
• Specific branding
• Information security marking - mandatory for US National System for Geospatial-Intelligence (NSG) users, otherwise optional
• High precision timestamps (per frame)
• Specific identifiers for Tracks
• Specific identifiers for Samples
• Specific identifiers for security metadata

There is also some specific metadata encoding required, but that isn't quite settled yet.