[filter-effects-1] Allow feTurbulence's reference implementation to compile

[AlanDeSmet]

The specification for the feTurbulenceElement filter primitive includes "C code below show[ing] the exact algorithm". Unfortunately, the code does not compile under any of the C or C++ standards supported by gcc 9.3.0. It has some very old C assumptions that are not legal C++ (and may generate warning in a modern C compiler), as well as some C++ casting operators and use of the bool type that are not legal C. Some relatively simple changes will bring it into compliance with modern C. My attempt to do so is attached.

turb.c.txt

[tateshelby28]

🙂