Improve culling data reuse

[paroj]

similar to what is suggested in the 2.0 slides p. 45 and following.

The given example is:

    target_output
    {
        pass render_scene
        {
            first_render_queue 10
            last_render_queue 90
        }

        pass render_quad { }

        pass render_scene
        {
            first_render_queue 91
            last_render_queue 95
        }
    }

however, this was never an issue. The RenderTarget is only updated once and both render_scene passes affect the per-target render-queue mask. The render_quad pass is sorted-in accordingly.

On the other hand:

        target scene
        {
            pass clear { }
            pass render_scene
            {
                first_render_queue 0
                last_render_queue 10
            }
        }

        target scene
        {
            pass render_scene
            {
                first_render_queue 10
                last_render_queue 20
            }
        }

will update the RenderTarget twice and hence do the culling again.

If neither viewport nor camera changed between target calls, we can skip _findVisibleObjects.

Possible gains: