message: Add clear method to CanvasRenderer
  * Add clear method to CanvasRenderer
  WebGLRenderer has the ```clear``` method , I think CanvasRenderer should has it too.
  Sometimes, maybe we want to clear the canvas manually via ``` game.renderer.clear()``` .
  If  CanvasRenderer doesn't  has this method ,  there will be an error.
  * Update CanvasRenderer.js
diff: @@ -210,6 +210,28 @@ export default class CanvasRenderer extends SystemRenderer
  this.emit('postrender');
  }
  + /**
  + * Clear the canvas of renderer.
  + *
  + * @param {string} [clearColor] - Clear the canvas with this color, except the canvas is transparent.
  + */
  + clear(clearColor)
  + {
  + const context = this.context;
  +
  + clearColor = clearColor || this._backgroundColorString;
  +
  + if (!this.transparent && clearColor)
  + {
  + context.fillStyle = clearColor;
  + context.fillRect(0, 0, this.width, this.height);
  + }
  + else
  + {
  + context.clearRect(0, 0, this.width, this.height);
  + }
  + }
  +
  /**
  * Sets the blend mode of the renderer.
  *
  