Fix clobbering of overridden canvas methods

The method may be overridden again after we override,
as is done in the `hidpi-canvas-polyfill`. So if we
restore the method to the original as we see it, we'll
wipe out the polyfill.

Fixes EFForg#2657, EFForg#1226

src/js/contentscripts/fingerprinting.js

@@ -204,9 +204,16 @@ function getFpPageScript() {
       );
 
       item.obj[item.propName] = (function (orig) {
+        // set to true after the first write, if the method is not
+        // restorable. Happens if another library also overwrites
+        // this method.
+        var skipMonitoring = false;
 
         function wrapped() {
           var args = arguments;
+          if (skipMonitoring) {
+            return orig.apply(this, args);
+          }
 
           if (is_canvas_write) {
             // to avoid false positives,
@@ -237,7 +244,13 @@ function getFpPageScript() {
             // optimization: one canvas write is enough,
             // restore original write method
             // to this CanvasRenderingContext2D object instance
-            this[item.propName] = orig;
+            // Careful! Only restorable if we haven't already been replaced 
+            // by another lib, such as the hidpi polyfill
+            if (this[item.propName] === wrapped) {
+              this[item.propName] = orig;
+            } else {
+              skipMonitoring = true;
+            }
           }
 
           return orig.apply(this, args);