Fix for bug reported by Pablo Vasquez (venomgfx) on IRC: Single layer

renders were broken.

This was caused by rB1a79abdad2443ff9f12e7efd95ee78a264a9d60a which
makes a copy of the render layer list for thread safety. The single
layer passed to this function is still in the original list though, so
to get the correct index it has to be looked up there. Otherwise no
active index is set and all layers are rendered every time.

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

@@ -588,7 +588,7 @@ void RE_InitState(Render *re, Render *source, RenderData *rd, SceneRenderLayer *
 	}
 	
 	if (srl) {
-		int index = BLI_findindex(&re->r.layers, srl);
+		int index = BLI_findindex(&rd->layers, srl);
 		if (index != -1) {
 			re->r.actlay = index;
 			re->r.scemode |= R_SINGLE_LAYER;