alphablending: Improve critical regions
alphablending: Improve critical regions

Change-Id: Ia1551fded4a84302a1afd491be0dcedeea788895

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