droidcamsrc: implement gst_droidcamsrc_stream_window_cancel_buffer
droidcamsrc: implement gst_droidcamsrc_stream_window_cancel_buffer

This is used by HAL to inform us that it wants to release a non-full buffer
and we should unref that buffer to avoid leaking it.

mlehtima/gst-droid,sledges/gst-droid,foolab/gst-droid,sailfishos/gst-droid,mlehtima/gst-droid,sledges/gst-droid,foolab/gst-droid,sledges/gst-droid