proper way to change element's caps in pipeline on runtime

[Byunghun Lee]

Hi dev.
Now I'm trying to change framerate of rtsp stream on runtime.
My test pipeline is like below.

rtspsrc ! depayloader ! decoder ! videorate ! video/x-raw,framerate=15/1 !
f.sink_1 \
rtspsrc ! depayloader ! decoder ! videorate ! video/x-raw,framerate=15/1 !
f.sink_2 \
funnel name=f ! videoscale ! videoconvert !
video/x-raw,width=640,height=384,format=RGB ! tensor_converter ! fakesink

After running pipeline 20sec,
I changed two capsfilter "video/x-raw,framerate=15/1" to
"video/x-raw,framerate=1/1".
Other elements reconfigured well after changing capsfilter, But the third
party plugin element "tensor_converter" doesn't perform reconfiguring
process.

By tracing logs of tensor_conveter, I saw that tensor_converter get
sink_query from capsfilter as framerate 1/1 but, cannot make any
intersected caps.
below lines are part of source code of tensor_converter.
```
static GstCaps *
gst_tensor_converter_query_caps (GstTensorConverter * self, GstPad * pad,
    GstCaps * filter)
{
  GstCaps *caps;
  caps = gst_pad_get_current_caps (pad);

  if (!caps) {
    caps = gst_pad_get_pad_template_caps (pad);
  }

  if (pad == self->sinkpad) {
    GstCaps *media_caps;

    media_caps = gst_tensor_converter_get_possible_media_caps (self);
    if (media_caps) {
      /* intersect with pad caps */
      GstCaps *tmp = gst_caps_intersect_full (media_caps, caps,
          GST_CAPS_INTERSECT_FIRST);
      gst_caps_unref (caps);
      caps = tmp;

      gst_caps_unref (media_caps);
    }
  }

  silent_debug_caps (self, caps, "caps");
  silent_debug_caps (self, filter, "filter");

  if (filter) {
    GstCaps *intersection;
    intersection =
        gst_caps_intersect_full (filter, caps, GST_CAPS_INTERSECT_FIRST);

    gst_caps_unref (caps);
    caps = intersection;
  }

  silent_debug_caps (self, caps, "result");
  return caps;
}
```
log says  caps = gst_pad_get_current_caps (pad); return old caps
and media_caps = NULL, filter = new_caps
and then gst_caps_intersect_full return EMPTY.

So I wonder what should I do to change caps.

And Is there any other examples of caps negotiation?
I'm trying to trace negotiation call stacks but It little bit confusing me.

Regards.
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