Move to private variables

Author: dmitrylyzo

src/subtitles-octopus.js

@@ -63,6 +63,10 @@ var SubtitlesOctopus = function (options) {
 
     self.hasAlphaBug = false;
 
+    // private
+    var targetWidth;    // Width of render target
+    var targetHeight;   // Height of render target
+
     (function() {
         if (typeof ImageData.prototype.constructor === 'function') {
             try {
@@ -753,11 +757,11 @@ var SubtitlesOctopus = function (options) {
             self.canvas.style.pointerEvents = 'none';
         }
 
-        if (self.canvas._width !== width || self.canvas._height !== height) {
+        if (targetWidth !== width || targetHeight !== height) {
             self.canvas.width = width;
             self.canvas.height = height;
-            self.canvas._width = width;
-            self.canvas._height = height;
+            targetWidth = width;
+            targetHeight = height;
 
             self.worker.postMessage({
                 target: 'canvas',