Add 20

With improved over setting NULL to freed variables in close() from 19.

20_forcefeedback/.gitignore
20_forcefeedback/LButton.c
20_forcefeedback/LButton.h
20_forcefeedback/LTexture.c
20_forcefeedback/LTexture.h
20_forcefeedback/Makefile
20_forcefeedback/README.md
20_forcefeedback/common.c
20_forcefeedback/common.h
20_forcefeedback/haptic.c
20_forcefeedback/splash.png

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have to go back to use xcode 9.4 to build sdl2 on mojave to get rid of initial black screen on mojave.