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)
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