Change SDL wrapper symbol names to better match actual SDL

  This follow the pattern used in OpenGL wrappers, that still remove
the "SDL_" prefix and use "sdl."
  Two new namespaces were added to also help better match SDL naming
pattern, that are "ttf." for "TTF_" functions and symbols, and "mix."
for "Mix_" prefixes.
  The change is expected to help in making it easier to look for SDL
documentation and easily match the wrapped interface.