Slope collision algorithm rewrite #2778

Job	Run time
clickable (Debug, amd64, glew)	10m 54s
clickable (Debug, amd64, sdl)	10m 8s
clickable (Debug, armhf, glew)	10m 29s
clickable (Debug, armhf, sdl)	9m 28s
clickable (Release, amd64, glew)	11m 4s
clickable (Release, amd64, sdl)	10m 13s
clickable (Release, arm64, glew)	11m 11s
clickable (Release, arm64, sdl)	10m 8s
clickable (Release, armhf, glew)	9m 35s
clickable (Release, armhf, sdl)	9m 14s
	1h 42m 24s

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