Cycles: multiple importance sampling for lamps, which helps reduce noise for

big lamps and sharp glossy reflections. This was already supported for mesh
lights and the background, so lamps should do it too.

This is not for free and it's a bit slower than I hoped even though there is
no extra BVH ray intersection. I'll try to optimize it more later.

* Area lights look a bit different now, they had the wrong shape before.
* Also fixes a sampling issue in the non-progressive integrator.
* Only enabled for the CPU, will test on the GPU later.
* An option to disable this will be added for situations where it does not help.

Same time comparison before/after:
http://www.pasteall.org/pic/show.php?id=43313
http://www.pasteall.org/pic/show.php?id=43314

intern/cycles/kernel/osl/osl_services.cpp

@@ -605,7 +605,11 @@ bool OSLRenderServices::get_object_standard_attribute(KernelGlobals *kg, ShaderD
 		return set_attribute_int(3, type, derivatives, val);
 	}
 	else if ((name == u_geom_trianglevertices || name == u_geom_polyvertices)
+#ifdef __HAIR__
 		     && sd->segment == ~0) {
+#else
+		) {
+#endif
 		float3 P[3];
 		triangle_vertices(kg, sd->prim, P);
 
@@ -675,7 +679,11 @@ bool OSLRenderServices::get_attribute(void *renderstate, bool derivatives, ustri
 	else {
 		object = sd->object;
 		prim = sd->prim;
+#ifdef __HAIR__
 		segment = sd->segment;
+#else
+		segment = ~0;
+#endif
 
 		if (object == ~0)
 			return get_background_attribute(kg, sd, name, type, derivatives, val);