fixed method call in basic sample. fixes #14

[jwoertink]

This fixes the missing overload error:

Overloads are:
 - SDL::Surface#fill(rect, r, g, b, a = nil)

  window.surface.fill(255, 255, 255)

[ysbaddaden]

But, Surface#fill(r, g, b, a) exists. I guess the compiler can't find it, and the #fill(rect, r, g, b, a) should be modified like so rect : Rect?

[jwoertink]

I think it doesn't catch that one because of these two lines calling the pointerof(rect). In this case, rect can't be nil, which, it would be in the Surface#fill(r, g, b, a).

in src/surface.cr:125: argument 'rect' of 'LibSDL#fill_rect' must be Pointer(SDL::Rect), not Pointer(Nil)

        LibSDL.fill_rect(self, pointerof(rect), color(r, g, b))

Unless that's a whole separate issue? What if I just add a default SDL::Rect to this line instead of passing nil. It would be SDL::Rect[0, 0, width, height]