imageoverlay: Handle alpha channel in  images
imageoverlay: Handle alpha channel in  images

Change-Id: Ie01d643bd4f5bc2e852f5b1e4195d02c85297965

shelsonjava/kms-filters,Kurento/kms-filters,TribeMedia/kms-filters,TribeMedia/kms-filters,shelsonjava/kms-filters,shelsonjava/kms-filters,Kurento/kms-filters,TribeMedia/kms-filters,Kurento/kms-filters