Add GetNextFrame timeout to CvSink and MjpegServer.

MjpegServer uses the timeout to generate keep-alives to any clients
(which helps detect disconnects and avoid stale client threads).

CvSink GrabFrame now defaults to a timeout, but the timeout can be
changed by the user, or the old no-timeout version is now available
as GrabFrameNoTimeout.

src/SourceImpl.h

@@ -82,6 +82,10 @@ class SourceImpl {
   // Blocking function that waits for the next frame and returns it.
   Frame GetNextFrame();
 
+  // Blocking function that waits for the next frame and returns it (with
+  // timeout in seconds).  If timeout expires, returns empty frame.
+  Frame GetNextFrame(double timeout);
+
   // Force a wakeup of all GetNextFrame() callers by sending an empty frame.
   void Wakeup();