fmp4mux: Fixed caps change handling for multiple streams
- Moved FKU code into separate function.
- Send FKU for each stream where the caps change happened.
- Send FKU for each stream where an incomplete GOP has been pushed on caps change.
- Push incomplete GOP from stream if the early fragment end would not contain that stream otherwise.
- If an incomplete GOP has been pushed on the previous fragment accept inter-frames on the next fragment and do not drop them.