make color masks always uint32s (as in the C API)

I have this code:
```nimrod
  const # ARGB32
    amask = 0xff000000
    rmask = 0x00ff0000
    gmask = 0x0000ff00
    bmask = 0x000000ff

  let surface = sdl2.createRGBSurfaceFrom(cImage, w, h, 32, stride,
                                         rmask, gmask, bmask, amask)
```
It failed because the mask parameters are ints instead of unsigned ints, like in the C API.