gst_vdp_output_src_pad_push, gst_vdp_output_src_pad_alloc_buffer and
gst_vdp_output_src_pad_get_device now take a GError parameter to be able to
signal errors to the caller
GstVdpOutputSrcPad is a subclass of GstPad that is used for pushing out
GstVdpOutputBuffers. It takes care of caps negotiation and conversion to
video/x-raw-rgb.