v4l2src capture/render with single png image captures (dangling reference)

[Steve Evers]

Thanks Nicolas

I am using GStreamer 1.0. The target device is an IMX.6.

I will look at placing the imaging portion of the pipeline under appsrc control.

> Maybe worth on which version of GStreamer this is based on. The
> references that matters is the device FD that may still be Open. The
> most common cause is when a buffer is leaked (or still being used in
> your pipeline). If ising GStreamer 1.4+, trace the v4l2allocator
> section, and watch for finalize function to be called. If that does not
> occure, you have a buffer leak. Sometimes, in dynamic pipeline, it may
> happen late, which can be solve by draining the pipeline.
> 
> A note on the method to take snapshot. Using valve is a bit racy and
> require extra work, which may lead issues like you are facing. The
> method I usually recommend is to enable the last-sample property on the
> sink. To capture, create a separate encoding pipeline with an appsrc,
> read the last-sample and push it through the appsrc.