This way we'll reuse the GstVdp[Video|Output]Buffers if they're of the same
size and chroma-type/rgba-format.
Also remove gst_vdp_output_src_pad_negotiate and set a "setcaps" function on
GstVdpOutputSrcPad instead, leaving negotiation to GstVdpVideoPostProcess.
instead we do as GstVdpVideoSrcPad and use the "templ" property of GstPad,
which enable us to change the signature of gst_vdp_output_src_pad_new to match
gst_pad_new_from_template
we now no longer try to get the GstVdpDevice from downstream since it in
practice didn't give us anything and complicates the code alot. Nevertheless if device
distribution should be done there's probably a lot better ways to do it.
Instead we now simply aquire the device in vdpauvideopostprocess when we're
going into PAUSED.
