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