Make a local variable of ctx

lib/torque/renderer/point.js

@@ -88,13 +88,13 @@ var CartoDatasource = require('./datasource');
         var canvas = this._canvas;
         var color = this._Map['-torque-clear-color']
         // shortcut for the default value
+        var ctx = this._ctx;
         if (color  === "rgba(255, 255, 255, 0)" || !color) {
-          this._ctx.save();
-          this._ctx.setTransform(1, 0, 0, 1, 0, 0);
-          this._ctx.clearRect(0, 0, canvas.width, canvas.height);
-          this._ctx.restore();
+          ctx.save();
+          ctx.setTransform(1, 0, 0, 1, 0, 0);
+          ctx.clearRect(0, 0, canvas.width, canvas.height);
+          ctx.restore();
         } else {
-          var ctx = this._ctx;
           ctx.setTransform(1, 0, 0, 1, 0, 0);
           var compop = this._Map['comp-op']
           ctx.globalCompositeOperation = compop2canvas(compop) || compop;