Rendering optimizations

[impworks]

Rendering performance depends on the order of rendering object. Less spritebatches and render target switches is always better. Therefore, the engine should have an ability to reorder object graph for rendering.

Here's an example scene graph:

Naive rendering order causes many render target switches:

1. RT = 0
2. RT = 1
3. Render 1
4. Render 2
5. RT = 0
6. Render RT 1
7. RT = 2
8. Render 3
9. Render 4
10. RT = 3
11. Render 5
12. Render 6
13. RT = 2
14. Render RT 3
15. Render 7
16. RT = 0
17. Render RT 2
18. Render 8

The optimal order would be:

1. RT = 1
2. Render 1
3. Render 2
4. RT = 3
5. Render 5
6. Render 6
7. RT = 2
8. Render 3
9. Render 4
10. Render RT 3
11. Render 7
12. RT = 0
13. Render RT 1
14. Render RT 2
15. Render 8

Benefits: each 4 RT switches instead of one. Each RT is used exactly once, which means default RenderTargetUsage can be used instead of the slow RenderTargetUsage.PreserveContents.