js: fix subtitles disappear when resizing

1. Missing `px` in offset comparison - deferred resizing resets
the canvas by writing the same values to `width` and `height`.
[turuslan <turuslan.devbox@gmail.com>] found this bug.

2. Since RenderAhead resizes the canvas according to the
rendered frame, the native properties will be overwritten, and
we need additional properties for comparison.

Author: dmitrylyzo

src/subtitles-octopus.js

@@ -742,26 +742,22 @@ var SubtitlesOctopus = function (options) {
             return;
         }
 
-
-        if (
-          self.canvas.width != width ||
-          self.canvas.height != height ||
-          self.canvas.style.top != top ||
-          self.canvas.style.left != left
-        ) {
+        if (videoSize != null) {
+            self.canvasParent.style.position = 'relative';
+            self.canvas.style.display = 'block';
+            self.canvas.style.position = 'absolute';
+            self.canvas.style.width = videoSize.width + 'px';
+            self.canvas.style.height = videoSize.height + 'px';
+            self.canvas.style.top = top + 'px';
+            self.canvas.style.left = left + 'px';
+            self.canvas.style.pointerEvents = 'none';
+        }
+
+        if (self.canvas._width !== width || self.canvas._height !== height) {
             self.canvas.width = width;
             self.canvas.height = height;
-
-            if (videoSize != null) {
-                self.canvasParent.style.position = 'relative';
-                self.canvas.style.display = 'block';
-                self.canvas.style.position = 'absolute';
-                self.canvas.style.width = videoSize.width + 'px';
-                self.canvas.style.height = videoSize.height + 'px';
-                self.canvas.style.top = top + 'px';
-                self.canvas.style.left = left + 'px';
-                self.canvas.style.pointerEvents = 'none';
-            }
+            self.canvas._width = width;
+            self.canvas._height = height;
 
             self.worker.postMessage({
                 target: 'canvas',