Cycles: Reduce memory used by background light update

Simple change: just get rid of intermediate data a bit earlier, before
final pixels array is being allocated. This gives around 30% of memory
save during light update (this is about 60meg in the frank sheep file
i'm using here).

This isn't really visible by artists a lot, because actual spike happens
on BVH construction. But it doesn't mean we shouldn't be accurate with
memory usage in other areas.

intern/cycles/render/light.cpp

@@ -82,6 +82,8 @@ static void shade_background_pixels(Device *device, DeviceScene *dscene, int res
 	device->mem_free(d_input);
 	device->mem_free(d_output);
 
+	d_input.clear();
+
 	float4 *d_output_data = reinterpret_cast<float4*>(d_output.data_pointer);
 
 	pixels.resize(width*height);