compositemixer: Fix race condition recalculating sizes
compositemixer: Fix race condition recalculating sizes

Change-Id: I48ffc41ce82bbf3bf3ad4f46fa66528476e9a49d

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