main: Redo buffer handling in camera-gstreamer

main: Redo buffer handling in camera-gstreamer

Using this time a buffer list, similar to what simple-shm is doing to avoid
getting back that a buffer is busy, due to *not* prunning older buffers.
As such, double buffering seems to be sufficient so we don't need to
resort to having 3 buffers.

This should fix switching back and forth between other applications and
the camera application. Tested with vivid module.

Bug-AGL: SPEC-4884
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: Ia3987782b3e107a0055f9680fecfed0f9a96eec6