aggregator: Only send events up to CAPS event from gst_aggregator_set_src_caps()

aggregator: Only send events up to CAPS event from gst_aggregator_set_src_caps()

Otherwise setting the srcpad caps based on the sinkpad caps event will
already push a segment event downstream before the upstream segment is
known.

If the upstream segments are just forwarded when the upstream segment
event arrives this would result in two segment events being sent
downstream, of which the first one will usually be simply wrong.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2372>