fix SDL_version type error

``SDL_version`` was defined in SDL_version.h, and the field type is uint8_t, instead of int.
```c
typedef struct SDL_version
{
    Uint8 major;        /**< major version */
    Uint8 minor;        /**< minor version */
    Uint8 patch;        /**< update version */
} SDL_version;
```
Current sdl2_image version is 2.0.0, makes uint8_t and int returning same result under little endian.
When I copied that code to my rust-sdl2_ttf repo, I got a wrong version number. As current sdl2_ttf version is 2.0.12.

BTW: this pull request also makes SDLImageVersion field public.