Problem: artist wants character to walk in grass, but still have all rendered in seperate render-layers, for postpro effects and vblur. How to efficiently create a mask image you can put *over* the character for the grass? Solution has two parts; this commits allows any layer inside of the renderlayers to become a Z-mask (Z values for solid gets filled in, but not rendered). Second part of commit is render option "Only render stuff that's in front of a zbuffer value that was filled in (saves render time)