addShaderPass(shaderPass) {
        if(!shaderPass._isScenePass) {
            this.stacks.renderPasses.push(shaderPass);
            this.sortShaderPassStack(this.stacks.renderPasses);
        }
        else {
            this.stacks.scenePasses.push(shaderPass);
            this.sortShaderPassStack(this.stacks.scenePasses);
        }
    }
Add a shader pass to the stack

     params:
     @shaderPass (ShaderPass object): shaderPass to add to our scene
*