decodebin3: Fix global group-id handling

decodebin3: Fix global group-id handling

The goal of the "global" group-id is to fix new inputs that do not come from the
same "source" as others. In order to ensure all "current" streams have the same
group-id we distribute the first valid group-id to all streams.

This commit fixes two issues with that:

* When inputs are unlinked they weren't always properly resetted (it would only
  work if parsebin is used, which is no longer the default in
  uridecodebin3/playbin3).

* When computing the global group-id, take into account unset
  group-id (i.e. GST_GROUP_ID_INVALID).

Fixes https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/1698

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3712>