Fix [#21874] Sun/ Sky breaks when sun is in the night rotation

Disable atmosphere when the sun lamp direction is below the horizon. This 
feature needs a re-code...

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

@@ -739,10 +739,15 @@ static void atm_tile(RenderPart *pa, RenderLayer *rl)
 						if(*zrect >= 9.9e10 || rgbrect[3]==0.0f) {
 							continue;
 						}
-						
+												
 						if((lar->sunsky->effect_type & LA_SUN_EFFECT_AP)) {	
 							float tmp_rgb[3];
 							
+							/* skip if worldspace lamp vector is below horizon */
+							if(go->ob->obmat[2][2] < 0.f) {
+								continue;
+							}
+							
 							VECCOPY(tmp_rgb, rgbrect);
 							if(rgbrect[3]!=1.0f) {	/* de-premul */
 								float div= 1.0f/rgbrect[3];