Revert "package/gstreamer1/gst1-plugins-bad: disable opencv plugin with opencv-3"

This (partially) reverts commit 5e238a87.

The dependency is changed from a 'select' to a 'depends on' to avoid a
circular dependency caused by the introduction of OpenCV-3. This means
we can also drop the threads and C++ dependencies, since they are now
inherited via the depends on OpenCV.

Cc: Jonathan Ben Avraham <yba@tkos.co.il>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Samuel Martin <s.martin49@gmail.com>
[yann.morin.1998@free.fr: fix dependencies]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>