fixes crash caused by race derefing pixelbufferpool ivar while being destroyed and replaced by format change

removes cached pixelbufferpool and instead retrieves current pool from the compressionSession each time (as recommended by apple docs)

Bug: webrtc:14688
Change-Id: I2244e69e7f32b912021db0905b9d5867d0bf6357
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/284240
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Commit-Queue: Kári Helgason <kthelgason@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#38920}
diff --git a/AUTHORS b/AUTHORS
index bd7ab24..f79b6ad 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -86,6 +86,7 @@
 Mike Gilbert <floppymaster@gmail.com>
 Mike Wei <Mike.WeiB@gmail.com>
 Min Wang <mingewang@gmail.com>
+Mike Woodworth <mike@divergentmedia.com>
 Mo Zanaty <mzanaty@cisco.com>
 Nico Schlumprecht <me@github.nico.onl>
 Niek van der Maas <mail@niekvandermaas.nl>