Minor tweaks to video source frameworks

Fix the video source to close if file is open already and add a limit.

Change-Id: I36ada4c609d027b6eaa9b447fe9ad4115532edc1

test/video_source.h

@@ -57,6 +57,9 @@ class VideoSource {
 
   // Get the current frame counter, starting at 0.
   virtual unsigned int frame() const = 0;
+
+  // Get the current file limit.
+  virtual unsigned int limit() const = 0;
 };
 
 
@@ -94,6 +97,8 @@ class DummyVideoSource : public VideoSource {
 
   virtual unsigned int frame() const { return frame_; }
 
+  virtual unsigned int limit() const { return limit_; }
+
   void SetSize(unsigned int width, unsigned int height) {
     if (width != width_ || height != height_) {
       vpx_img_free(img_);