optimize backbuffer update #188

Job	Run time
build-arduino (release-v4.4, weather)	2m 36s
build (release-v4.4, calibration_helper)	2m 10s
build (release-v4.4, demo)	2m 11s
build (release-v4.4, dragon)	2m 4s
build (release-v4.4, grayscale_test)	2m 9s
build (release-v4.4, www-image)	2m 13s
build (release-v5.1, calibration_helper)	2m 7s
build (release-v5.1, demo)	2m 1s
build (release-v5.1, dragon)	2m 9s
build (release-v5.1, grayscale_test)	2m 0s
build (release-v5.1, www-image)	2m 8s
build (release-v5.1, screen_diag)	2m 1s
	25m 49s

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