add a write_image early out for single-block cases
add a write_image early out for single-block cases

fengzhyuan/visionworkbench,AveRapina/visionworkbench,DougFirErickson/visionworkbench,AveRapina/visionworkbench,fengzhyuan/visionworkbench,AveRapina/visionworkbench,AveRapina/visionworkbench,fengzhyuan/visionworkbench,DougFirErickson/visionworkbench,AveRapina/visionworkbench,fengzhyuan/visionworkbench,fengzhyuan/visionworkbench,DougFirErickson/visionworkbench,fengzhyuan/visionworkbench,DougFirErickson/visionworkbench,AveRapina/visionworkbench,fengzhyuan/visionworkbench,DougFirErickson/visionworkbench,DougFirErickson/visionworkbench