Fix mask combine filter bug

[hightopo]

In the demo (code below), i create a group with a rect mask, and create a sprite as group's child with a blurXY filter, then nothing is displayed.

So i hack WebGLFilterManager's pushFilter and popFilter method, in pushFilter method i create a new stencilManager instance and save the old one, in popFilter i destroy the new stencilManager and restore the old one, then this workaround works rightly as i expect.

I think this is a bug for PIXI, so i hope @photonstorm or @englercj can help me checking whether this fixing is right or not, thanks.

var game = new Phaser.Game(800, 600, Phaser.WEBGL, 'phaser-example', { preload: preload, create: create });

function preload() {
    game.load.image('phaser', 'assets/sprites/phaser2.png');
    game.load.script('filterX', '../filters/BlurX.js');
    game.load.script('filterY', '../filters/BlurY.js');
}

function create() {
    var logo = game.add.sprite(game.world.centerX, game.world.centerY, 'phaser');
    logo.anchor.setTo(0.5, 0.5);

    var blurX = game.add.filter('BlurX');
    var blurY = game.add.filter('BlurY');

    logo.filters = [blurX, blurY];
    var group = game.add.group();
        group.addChild(logo);
        group.mask = game.add.graphics();
        group.mask.beginFill(0xff);
        group.mask.drawRect(0,0,500,500);
}

After i hack WebGLFilterManager's code, i can get the right effect like below: