This is done by adding a `Unsafe_` prefix to `(const) char *` (C# `byte*`) functions.
I am using `string?` instead of `ReadOnlyMemory<byte>?` because the returned pointer can
get invalidated and cause memory-safety issues.
The returned pointer is automatically freed if the return type is `char *`.
I've checked that the documentation for all functions (except in SDL_stdinc.h)
requires calling `SDL_free()` on the pointer.
Susko3