Fix gl2d multiple subplot

Author: dy

src/plots/gl2d/convert.js

@@ -86,7 +86,7 @@ function Axes2DOptions(scene) {
         [0, 0, 0, 1]
     ];
 
-    this.borderColor = [0, 0, 0, 0];
+    this.borderColor = false;
     this.backgroundColor = [0, 0, 0, 0];
 
     this.static = this.scene.staticPlot;

src/plots/gl2d/scene2d.js

@@ -118,6 +118,7 @@ proto.makeFramework = function() {
 
         var gl = getContext({
             canvas: liveCanvas,
+            preserveDrawingBuffer: true,
             premultipliedAlpha: true
         });