function Compositor() {
    injectCSS();

    this._contexts = {};
    this._outCommands = [];
    this._inCommands = [];
    this._time = null;

    this._resized = false;

    var _this = this;
    window.addEventListener('resize', function() {
        _this.onResize();
    });
}
Instantiates a new Compositor.
The Compositor receives draw commands frm the UIManager and routes the to the
respective context objects.

Upon creation, it injects a stylesheet used for styling the individual
renderers used in the context objects.

@class Compositor
@constructor
@return {undefined} undefined