Phase one of better masking support while rendering.

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)

source/blender/render/intern/source/pipeline.c

@@ -505,6 +505,7 @@ static RenderResult *new_render_result(Render *re, rcti *partrct, int crop, int
 		
 		strcpy(rl->name, srl->name);
 		rl->lay= srl->lay;
+		rl->lay_zmask= srl->lay_zmask;
 		rl->layflag= srl->layflag;
 		rl->passflag= srl->passflag;
 		rl->pass_xor= srl->pass_xor;