encoder channel remap: improvements #200

Job	Run time
retag-continuous	4s
run Windows (legacy, GPUJPEG-Windows, 35, https://developer.download.nvidia.com/compute/cuda/11.8...	24m 53s
run Ubuntu (legacy)	43s
run Ubuntu (all)	27s
run Windows (all, GPUJPEG-Windows-all, all, https://developer.download.nvidia.com/compute/cuda/12...	27m 45s
	53m 52s

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