fixed isom mux for streams with broken first DTS
fixed isom mux for streams with broken first DTS

gpac/gpac,gpac/gpac,gpac/gpac,gpac/gpac,rbouqueau/gpac,rbouqueau/gpac,gpac/gpac,rbouqueau/gpac,rbouqueau/gpac,rbouqueau/gpac,gpac/gpac,gpac/gpac,rbouqueau/gpac,rbouqueau/gpac,gpac/gpac,rbouqueau/gpac