Fix swimming on slopes #1711

Job	Run time
clickable (Debug, amd64, glew)	11m 30s
clickable (Debug, amd64, sdl)	12m 13s
clickable (Debug, armhf, glew)	11m 8s
clickable (Debug, armhf, sdl)	10m 16s
clickable (Release, amd64, glew)	13m 57s
clickable (Release, amd64, sdl)	10m 41s
clickable (Release, arm64, glew)	12m 51s
clickable (Release, arm64, sdl)	10m 51s
clickable (Release, armhf, glew)	11m 15s
clickable (Release, armhf, sdl)	9m 24s
	1h 54m 6s

Provide feedback