Fix swimming on slopes #1816

Job	Run time
clickable (Debug, amd64, glew)	11m 7s
clickable (Debug, amd64, sdl)	10m 39s
clickable (Debug, armhf, glew)	11m 34s
clickable (Debug, armhf, sdl)	13m 43s
clickable (Release, amd64, glew)	10m 53s
clickable (Release, amd64, sdl)	10m 38s
clickable (Release, arm64, glew)	11m 17s
clickable (Release, arm64, sdl)	11m 1s
clickable (Release, armhf, glew)	9m 48s
clickable (Release, armhf, sdl)	9m 17s
	1h 49m 57s

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