Add support for multisampled render textures

[dev7355608]

This change makes it possible to enable multisampling; it works with and without double buffering.

const layer = new PIXI.display.Layer();
layer.useRenderTexture = true;
layer.useDoubleBuffer = true;

const renderTexture = layer.getRenderTexture();
renderTexture.framebuffer.multisample = PIXI.MSAA_QUALITY.HIGH;

const layerSprite = new PIXI.Sprite(renderTexture);

// ...

[ivanpopelyshev]

good idea! though i dont remember whether double buffering works now, but blit there is good.