decoder: h264: fix tracking of DPB size changes.
decoder: h264: fix tracking of DPB size changes.

Add support for MVC streams with multiple SPS and subset SPS headers
emitted regularly, e.g. at around every I-frame. Track the maximum
number of views in ensure_context() and really reset the DPB size to
the expected value, always. i.e. even if it decreased. dpb_reset()
only cares of ensuring the DPB allocation.

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