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;

source/blender/render/intern/source/rendercore.c

@@ -897,7 +897,7 @@ void zbufshadeDA_tile(RenderPart *pa)
 			sdata.rl= rl;
 			sdata.psmlist= &psmlist;
 			sdata.edgerect= edgerect;
-			zbuffer_solid(pa, rl->lay, rl->layflag, make_pixelstructs, &sdata);
+			zbuffer_solid(pa, rl, make_pixelstructs, &sdata);
 			if(R.test_break()) break; 
 		}
 		
@@ -1063,7 +1063,7 @@ void zbufshade_tile(RenderPart *pa)
 		shade_sample_initialize(&ssamp, pa, rl);
 		addpassflag= rl->passflag & ~(SCE_PASS_Z|SCE_PASS_COMBINED);
 		
-		zbuffer_solid(pa, rl->lay, rl->layflag, NULL, NULL);
+		zbuffer_solid(pa, rl, NULL, NULL);
 		
 		if(!R.test_break()) {	/* NOTE: this if() is not consistant */
 			

source/blender/render/intern/source/zbuf.c

@@ -1925,7 +1925,7 @@ void zbufclip4(ZSpan *zspan, int obi, int zvlnr, float *f1, float *f2, float *f3
 
 /* ***************** ZBUFFER MAIN ROUTINES **************** */
 
-void zbuffer_solid(RenderPart *pa, unsigned int lay, short layflag, void(*fillfunc)(RenderPart*, ZSpan*, int, void*), void *data)
+void zbuffer_solid(RenderPart *pa, RenderLayer *rl, void(*fillfunc)(RenderPart*, ZSpan*, int, void*), void *data)
 {
 	ZbufProjectCache cache[ZBUF_PROJECT_CACHE_SIZE];
 	ZSpan zspans[16], *zspan; /* 16 = RE_MAX_OSA */
@@ -1935,7 +1935,9 @@ void zbuffer_solid(RenderPart *pa, unsigned int lay, short layflag, void(*fillfu
 	ObjectInstanceRen *obi;
 	ObjectRen *obr;
 	float winmat[4][4], bounds[4], ho1[4], ho2[4], ho3[4], ho4[4]={0};
+	unsigned int lay= rl->lay, lay_zmask= rl->lay_zmask;
 	int i, v, zvlnr, zsample, samples, c1, c2, c3, c4=0;
+	short layflag= rl->layflag;
 	short nofill=0, env=0, wire=0, all_z= layflag & SCE_LAY_ALL_Z;
 
 	samples= (R.osa? R.osa: 1);
@@ -2016,7 +2018,7 @@ void zbuffer_solid(RenderPart *pa, unsigned int lay, short layflag, void(*fillfu
 					}
 				}
 			}
-			else if(all_z) {
+			else if(all_z || (obr->lay & lay_zmask)) {
 				env= 1;
 				nofill= 0;
 				ma= NULL;