Make sure `Rectf` width and height never go below 0 #13

Job	Run time
clickable (Debug, amd64, glew)	21m 53s
clickable (Debug, amd64, sdl)	18m 2s
clickable (Debug, armhf, glew)	22m 0s
clickable (Debug, armhf, sdl)	15m 9s
clickable (Release, amd64, glew)	16m 12s
clickable (Release, amd64, sdl)	16m 17s
clickable (Release, arm64, glew)	19m 31s
clickable (Release, arm64, sdl)	16m 2s
clickable (Release, armhf, glew)	14m 51s
clickable (Release, armhf, sdl)	14m 16s
	2h 54m 13s

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