fmp4mux: Fix draining in chunk mode if keyframes are too late
requested to merge slomo/gst-plugins-rs:fmp4mux-chunking-fragment-start-before-chunk-start into main
We would create another chunk that ends after the fragment end, and would from then on consider the stream always filled for the chunk because it starts after the current fragment end (i.e. nothing would go into this fragment).
This is obviously wrong because the actual fragment end moved further ahead because of the additional chunk.