setOpacity(alpha) {
		if (typeof alpha === "number") {
			this.alpha = clamp(alpha, 0.0, 1.0);
			// Set to 1 if alpha is NaN
			if (isNaN(this.alpha)) {
				this.alpha = 1.0;
			}
			this.isDirty = true;
		}
	}
set the renderable alpha channel value<br>
@param {number} alpha - opacity value between 0.0 and 1.0