v4l2src: Avoid decide allocation on active pool
v4l2src: Avoid decide allocation on active pool

v4l2src will renegotiate only if the format have changed. As of now,
it's not possible to change the allocationw without resetting the
camera. To avoid unwanted side effect, simply keep the old allocation
if no renegotiation is taking place. This fixes assertion and possible
failures in USERPTR or DMABUF import mode (when using downstream pools).

https://bugzilla.gnome.org/show_bug.cgi?id=754042

Kurento/gst-plugins-good,Kurento/gst-plugins-good,Kurento/gst-plugins-good,Kurento/gst-plugins-good,Kurento/gst-plugins-good