PipeWire capture: fix camera capability enumeration race and duplicates

Don't enumerate capabilities without actual format changes. Push the new
discovered capabilities into a pending list and swap pending list into
actual capabilities only after all params from the enumeration have been
received to ensure capabilities are never empty while enumerating.

Bug: webrtc:478386505
Change-Id: Iad8d076de067d4c663b8b281cb65158a38fd1309
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/457941
Reviewed-by: Alexander Cooper <alcooper@chromium.org>
Commit-Queue: Jan Grulich <grulja@gmail.com>
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Reviewed-by: Andreas Pehrson <apehrson@mozilla.com>
Cr-Commit-Position: refs/heads/main@{#47290}