ImageDecoder cleanup. #2403

Job	Run time
Quick Checks	7s
windows-2019 / Debug	6m 42s
Documentation	12m 21s
ubuntu-22.04 / gcc / Debug	4m 48s
windows-2019 / RelWithDebInfo	6m 51s
ubuntu-22.04 / gcc / RelWithDebInfo	9m 45s
windows-2022 / Debug	5m 54s
ubuntu-20.04 / clang / Debug	3m 36s
windows-2022 / RelWithDebInfo	6m 55s
ubuntu-20.04 / clang / RelWithDebInfo	3m 31s
macos-12 / clang / Debug	3m 38s
macos-12 / clang / RelWithDebInfo	3m 42s
	1h 7m 50s

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