Attempt to clean-up/restore pending rendering operations on RenderTask.cancel (issue 10200)

Please note that, given the lack of a runnable example, I'm not totally sure if this first of all is enough to *completely* address the issue as filed and second of all if we actually want this new behaviour.

src/display/api.js

@@ -2439,6 +2439,9 @@ var InternalRenderTask = (function InternalRenderTaskClosure() {
     cancel: function InternalRenderTask_cancel() {
       this.running = false;
       this.cancelled = true;
+      if (this.gfx) {
+        this.gfx.endDrawing();
+      }
       if (this._canvas) {
         canvasInRendering.delete(this._canvas);
       }