plugins: check if the pool config is already set
plugins: check if the pool config is already set

In commit 97b768, a regression for GStreamer 1.2 was introduced:

GStreamer 1.2 doesn't check, in gst_buffer_pool_set_config() if the
config option is already set. This patch adds an inline function to
first verify if the option is not in the pool config berfore add it.

ceyusa/gstreamer-vaapi,CapOM/gstreamer-vaapi,CapOM/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,GStreamer/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,GStreamer/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,CapOM/gstreamer-vaapi,ceyusa/gstreamer-vaapi