Backport gstreamer patch that fixes videoconvert performance regression

[PerfectCarl]

gstreamer-1.0 has a well known performance regression that make videconvert needlessly CPU intensive in some cases.

It may impact video capturing and even in some case video playback.
(I don't know if the thumbnailing service uses gstreamer)

The issue is available here : https://bugzilla.gnome.org/show_bug.cgi?id=721701

The related commits are likely those ones:
  - http://cgit.freedesktop.org/gstreamer/gst-plugins-base/commit/?id=14b5999bca16d9ac18bdcd5905c472bec2fe247e
  - http://cgit.freedesktop.org/gstreamer/gst-plugins-base/commit/gst/videoconvert/videoconvert.c?id=14b5999bca16d9ac18bdcd5905c472bec2fe247e

[Sergey "Shnatsel" Davidoff (shnatsel)]

From the upstream bug report:

<Tim-Philipp Müller>
The 'target_milestone' field indicates where this is fixed. 1.3.1 means it's
only in master, not the 1.2 branch. The patches were too invasive to
cherry-pick IIRC.

We might want to check with GStreamer maintainers before backporting.