[Bug 739281] video-blend: correct improper logic calculations

[GStreamer (bugzilla.gnome.org)]

https://bugzilla.gnome.org/show_bug.cgi?id=739281
  GStreamer | gst-plugins-base | unspecified

Luis de Bethencourt <luis.bg> changed:

           What    |Removed                     |Added
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             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1

--- Comment #18 from Luis de Bethencourt <luis.bg at samsung.com> 2014-11-02 11:07:28 UTC ---
https://bugzilla.gnome.org/show_bug.cgi?id=739516 <- Is an other testcase.

Both Vineeth and my patch (in that bug) resolve the segfault. Vineeth catches
the negative values that will be problematic and sets the variables to 0. I
check for the value about to become negative and I go to fail.

What bothers me though is that neither of the two patches create a video where
the off frame overlay is not blended in, and all what is outputed is the
original. In both cases the frames are just dropped.

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