OCamlSDL2_TTF useful primitives

Hello,

I have added some primitives in order to use the library.
The text drawing primitives take an SDLColor struct that is not translated in OCamlSDL2, so I created a type color. Maybe this type should be in OCamlSDL2?
Also, I think it would be easier if all the code of OCamlSDL2_Image and OCamlSDL2_TTF were merged in the OCamlSDL2 repo, in order to simplify compilation of the project. What do you think about that?