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

Change-Id: I48ffc41ce82bbf3bf3ad4f46fa66528476e9a49d

shelsonjava/kms-elements,TribeMedia/kms-elements,shelsonjava/kms-elements,Kurento/kms-elements,TribeMedia/kms-elements,Kurento/kms-elements,Kurento/kms-elements,shelsonjava/kms-elements,TribeMedia/kms-elements