Halts a item at a given frame. The frame is provided as a percentage
of the whole transition.
function _haltItemAtFrame(item, perc) {
        item.mod.halt();
        item.halted = true;
        if (item.startSpec && (perc !== undefined)) {
            if ((item.startSpec.opacity !== undefined) && (item.endSpec.opacity !== undefined)) {
                item.mod.setOpacity(_interpolate(item.startSpec.opacity, item.endSpec.opacity, perc));
            }
            if (item.startSpec.transform && item.endSpec.transform) {
                var transform = [];
                for (var i = 0; i < item.startSpec.transform.length; i++) {
                    transform.push(_interpolate(item.startSpec.transform[i], item.endSpec.transform[i], perc));
                }
                item.mod.setTransform(transform);
            }
        }
    }