Clean up PNG and JPEG encoding properly #1358

Job	Run time
flash-samples	41s
docs	35s
android	26m 36s
air-samples (4.0.5)	3m 34s
air-samples (4.1.5)	3m 25s
html5-samples	1m 21s
ios	11m 50s
air-samples (4.2.5)	3m 38s
linux	14m 23s
air-samples (4.3.1)	3m 35s
macos	51m 18s
windows	14m 23s
package-haxelib	1m 49s
hashlinkc-samples (windows-latest)	1m 54s
neko-samples (3.4.7, windows-latest)	45s
hashlinkc-samples (ubuntu-20.04)	1m 33s
hashlink-samples (windows-latest)	43s
neko-samples (3.4.7, ubuntu-20.04)	15s
hashlinkc-samples (macos-12)	1m 40s
hashlink-samples (ubuntu-20.04)	18s
neko-samples (3.4.7, macos-12)	58s
hashlink-samples (macos-12)	42s
neko-samples (4.2.5, windows-latest)	41s
neko-samples (4.2.5, ubuntu-20.04)	20s
neko-samples (4.2.5, macos-12)	53s
notify	5s
	2h 27m 55s

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