Named texture in Grab Pass

Current refraction makes an un-named GrabPass which means every object is preceded by a grab. This is expensive. I'd like the option to specify a Grab texture, even if it's just a choice between per-object (as now) and global - one grab for the scene i.e. GrabPass{ "_GrabTexture" }


I am manually hacking this, but would be nice to have it as an option.