setup_source() tries to plug a typefind element unconditionally to the
source element on non-live streams, no matter if the element is dynamic
or not. In the former case, the element might not have any src pad
created, so the plugging will fail, triggering an unrecoverable error.
This patch only tries the plugging when the element is not dynamic (no
new-pad callback has been configured). In case the element is dynamic,
the callback will take care of configuration when pads appear in the
future.
This solves many regressions on the YouTube MSE Conformance Tests[1] at
least in downstream WPE 2.38[2] after migrating from GStreamer 1.16 to
1.18, a change that introduces Playbin 3.
[1] https://ytlr-cert.appspot.com/latest/main.html
[2] https://github.com/WebPlatformForEmbedded/WPEWebKit/tree/wpe-2.38
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/4074>
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