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:
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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).
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When computing the global group-id, take into account unset group-id (i.e. GST_GROUP_ID_INVALID).
Fixes #1698 (closed)