Protect the initial state  of `this._activeRenderTarget` in _indexContext (#3443)
Protect the initial state  of `this._activeRenderTarget` in _indexContext (#3443)

* Update WebGLRenderer.js

```
    this._activeRenderTarget = null;
    this._activeTextureLocation = 999;
    this._activeTexture = null;
````
must before `this._indexContext()` , otherwise  the _activeRenderTarget & _activeTexture init in  _indexContext will back to `null`.

* Add a blank line for cosmetic.

sfinktah/pixi.js,out-of-band/pixi.js,themoonrat/pixi.js,pixijs/pixi.js,bma73/pixi.js,Makio64/pixi.js,ceco-fmedia/pixi.js,jpweeks/pixi.js,bma73/pixi.js,jojuntune/pixi.js,englercj/pixi.js,lucap86/pixi.js,drkibitz/pixi.js,Cristy94/pixi.js,ceco-fmedia/pixi.js,jpweeks/pixi.js,staff0rd/pixi.js,Makio64/pixi.js,staff0rd/pixi.js,finscn/pixi.js,out-of-band/pixi.js,GoodBoyDigital/pixi.js,jojuntune/pixi.js,Cristy94/pixi.js,sfinktah/pixi.js,englercj/pixi.js,lucap86/pixi.js,finscn/pixi.js,drkibitz/pixi.js,themoonrat/pixi.js,pixijs/pixi.js