Fix imageconvert #183

Job	Run time
build-arduino (release-v4.4, weather)	3m 5s
build (release-v4.4, calibration_helper)	2m 8s
build (release-v4.4, demo)	2m 18s
build (release-v4.4, dragon)	2m 10s
build (release-v4.4, grayscale_test)	2m 7s
build (release-v4.4, www-image)	2m 14s
build (release-v5.1, calibration_helper)	2m 12s
build (release-v5.1, demo)	2m 13s
build (release-v5.1, dragon)	2m 15s
build (release-v5.1, grayscale_test)	2m 13s
build (release-v5.1, www-image)	2m 14s
build (release-v5.1, screen_diag)	2m 23s
	27m 32s

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