Backport of "playbin3: Add lock to protect buffering messages" into 1.20
Backport of !1840 (merged) "playbin3: Add lock to protect buffering messages".
Commits:
- 17f11c2c playbin3: Add lock to protect buffering messages
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Backport of !1840 (merged) "playbin3: Add lock to protect buffering messages".
Commits: