Use size_t instead of ptrdiff_t and reduce header usage.

This code is not of very good quality, but a few methods such as the
RGB/HSV/HSL/YUV conversions are still useful, so I’m deciding to keep
them for now.

This is because <cstdint> is not guaranteed to expose C99 types to the
global namespace, and we don’t want to go through the hassle of using
each of them one by one.

This implies creating a trimmed down branch that contains almost
nothing. It will then be usable by projects that do not need the
complex build system, and by the engine itself as a submodule from
the same repository.

This will probably end up upstream very soon:
    https://github.com/emscripten-ports/SDL2/pull/104

Does not work very well because of CORS restrictions in browsers.