code: function (clearAll, clearColor) {
          var dom = this.dom;
          var ctx = this.ctx;
          var width = dom.width;
          var height = dom.height;
  
          var clearColor = clearColor || this.clearColor;
          var haveMotionBLur = this.motionBlur && !clearAll;
          var lastFrameAlpha = this.lastFrameAlpha;
  
          var dpr = this.dpr;
  
          if (haveMotionBLur) {
              if (!this.domBack) {
                  this.createBackBuffer();
              }
  
              this.ctxBack.globalCompositeOperation = 'copy';
              this.ctxBack.drawImage(
                  dom, 0, 0,
                  width / dpr,
                  height / dpr
              );
          }
  
          ctx.clearRect(0, 0, width, height);
          if (clearColor && clearColor !== 'transparent') {
              var clearColorGradientOrPattern;
              // Gradient
              if (clearColor.colorStops) {
                  // Cache canvas gradient
                  clearColorGradientOrPattern = clearColor.__canvasGradient || Style.getGradient(ctx, clearColor, {
                      x: 0,
                      y: 0,
                      width: width,
                      height: height
                  });
  
                  clearColor.__canvasGradient = clearColorGradientOrPattern;
              }
              // Pattern
              else if (clearColor.image) {
                  clearColorGradientOrPattern = Pattern.prototype.getCanvasPattern.call(clearColor, ctx);
              }
              ctx.save();
              ctx.fillStyle = clearColorGradientOrPattern || clearColor;
              ctx.fillRect(0, 0, width, height);
              ctx.restore();
          }
  
          if (haveMotionBLur) {
              var domBack = this.domBack;
              ctx.save();
              ctx.globalAlpha = lastFrameAlpha;
              ctx.drawImage(domBack, 0, 0, width, height);
              ctx.restore();
          }
      }
docs: 清空该层画布
  @param {boolean} [clearAll]=false Clear all with out motion blur
  @param {Color} [clearColor]