Ensure that the clipping path is reset when the state is restored (issue 6413)

According to the specification, see `NOTE 2` in http://www.adobe.com/content/dam/Adobe/en/devnet/acrobat/pdfs/PDF32000_2008.pdf#G7.3882161, it appears that we should ensure that the clipping path is reset when the restore (`Q`) operator is encountered.

Fixes 6413.

src/display/canvas.js

@@ -970,6 +970,9 @@ var CanvasGraphics = (function CanvasGraphicsClosure() {
         this.current = this.stateStack.pop();
         this.ctx.restore();
 
+        // Ensure that the clipping path is reset (fixes issue6413.pdf).
+        this.pendingClip = null;
+
         this.cachedGetSinglePixelWidth = null;
       }
     },