Add new fmp4 plugin with muxers for ISO fragmented MP4, DASH and CMAF containers

TODO:

• Make playback work in GStreamer (gstreamer!1234 (merged))
• Test if AAC works correctly
• Figure out if the negative composition time offset and DTS shifting is correct, or how else to handle negative DTS without edit lists
• Write AVC/AAC/HEVC specific brands into ftyp/styp
• Double check against the CMAF and ISOBMFF specs and the "brand"/profile constraints
• More testing
• Maybe rename the plugin to fmp4mux or so. This can easily support other fragmented MP4 "brands" later
• Add a fmp4mux element factory that creates fragmented MP4 without the CMAF constraints and brands
• Check what can be done in qtdemux for fixing up DTS in case of frame reordering (see gstreamer!1294 (merged))
• Add some automated test to validate the buffer metadata generation / buffer-list chunking at least