compositemixer: Solve race conditions recalculating sizes
compositemixer: Solve race conditions recalculating sizes

Change-Id: I598d9f5059686429c0051b76bf9e43d1dacbb1f4

shelsonjava/kms-core,ESTOS/kms-core,ESTOS/kms-core,KurentoLegacy/gst-kurento-plugins,shelsonjava/kms-core,KurentoLegacy/gst-kurento-plugins,shelsonjava/kms-core,TribeMedia/kms-core,KurentoLegacy/gst-kurento-plugins,TribeMedia/kms-core,Kurento/kms-core,shelsonjava/kms-core,TribeMedia/kms-core,ESTOS/kms-core,Kurento/kms-core,Kurento/kms-core,Kurento/kms-core,KurentoLegacy/gst-kurento-plugins,TribeMedia/kms-core,ESTOS/kms-core