Add support for scene transition

[coeuvre]

Multiple scenes are common in a large game. We should add several built in transition effects. The basic idea of how to do transition are:

1. We have currently running scene from_scene and the scene to be transited to to_scene.
2. Render one frame of from_scene to a texture, and create a sprite from_sprite.
3. Assume that the to_scene have enough resources to render. We render one frame of to_scene to a texture, and crate a sprite to_sprite.
4. Change to a temporary scene which has only two sprites: from_sprite and to_sprite.
5. Perform transition between these two sprites.
6. Once the transition has finished, we change the scene to to_scene.

As you can see, the most important step is render to texture. So we should find a way to render scene to a texture first and then do the scene transition.

Any suggestions?