* Add a easy way to clear RenderTexture

In current PIXI, clear a RenderTexture is complex : 
```
renderer.bindRenderTexture(myRenderTexture);
renderer.clear();
// restore the previous renderTexture
renderer.bindRenderTexture();
```
And bind renderTexture is a high-cost operation . We shouldn't call it for cleaning render texture.

So I  add this method.

relatived : https://github.com/pixijs/pixi.js/issues/3595

* Add `return this`