videoaggregator: Fix locking around vagg->info

videoaggregator: Fix locking around vagg->info

Take `GST_OBJECT_LOCK` when writing `vagg->info`, so that reading in
subclasses is protected against races, as documented in the struct.

    /*< public >*/
    /* read-only, with OBJECT_LOCK */
    GstVideoInfo                  info;

`gst_video_aggregator_default_negotiated_src_caps` should take the
`GST_VIDEO_AGGREGATOR_LOCK` to avoid racing with
`gst_video_aggregator_reset` called by
`gst_video_aggregator_release_pad` of the last sinkpad. Otherwise it can
happen that `latency = gst_util_uint64_scale (...` gets called with a
zero framerate.

There doesn't seem to be any reason not to use the local `info` instead
of `vagg->info`, so do that.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/915>