devel/csfml: New port: C bindings to the Simple and Fast Multimedia Library (SFML)

The Simple and Fast Multimedia Library (SFML) exists in devel/sfml.
SFML is a C++ library, and as such, may introduce complexity for other
languages to use the library. CSFML is an official SFML project to
provide C bindings to SFML, with the aim of reducing language binding
complexity, to enable other languages to use C over C++.

Website: https://github.com/SFML/CSFML

Differential Revision: https://reviews.freebsd.org/D41519