updateTexture call bug fix

In PixiShader class, the updateTexture call always uses the
defaultRenderer as reference. However, if the user created more than one
WebGLRenderer instance, it will be wrong.

So, I added an instances array variable under the PIXI.WebGLRenderer and
use the gl.id as index to make sure the updateTexture call works
properly.

Related issue: #1122

Before update:
http://jsfiddle.net/glafarge/6h84bpnk/1/

After update:
http://jsfiddle.net/6h84bpnk/3/

The second instance after the update not working due to another bug in EventTarget.js.