[Bug 740557] videotestsrc: add ball motion based on system clock

[GStreamer (bugzilla.gnome.org)]

https://bugzilla.gnome.org/show_bug.cgi?id=740557
  GStreamer | gst-plugins-base | git

--- Comment #15 from carl at personnelware.com 2014-11-29 20:45:14 UTC ---
why wallclock: 

as a human observing and comparing things I have some easy reference about what
correct is.   Like when my flip was happening different from clockoverlay
time-format="%S" incrementing, I put the gstreamer ximagesink window next to my
desktop clock display and quickly verified that they were both clicking seconds
at the same time.

I can't use current running time because I have more than one os process
generating frames, possibly on different machines (and yes, I understand
keeping clocks in synk is an issue).   I can use the clock on my cell phone to
verify the 2 OS clocks are about right.  

I opened with "as a human" because I am bumping up against too many things for
my brain to reconcile: like when my sweep was upside down, I tried to think "it
doesn't matter where 0 is, I can watch the 2 displays and see if they were
hitting 0 at the same time."  It was distracting because I am used to it being
at the top of the circle, so it was like both were broken by the same amount. 
given the point of this code is testing, it is going to be used when things may
not be working right, and it is up to a human to get to the bottom of what is
broken.  adding something to think about is bad.

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