gst/videotestsrc/gstvideotestsrc.*: Add a "blink" pattern.  Turn on the pain.  Apologies.  It's useful for testing ve...

Original commit message from CVS:
* gst/videotestsrc/gstvideotestsrc.c:
* gst/videotestsrc/gstvideotestsrc.h:
Add a "blink" pattern.  Turn on the pain.  Apologies.  It's useful
for testing vertical refresh synchronization.