wined3d: Add some 'fall through' comments (coverity)

[Henri Verbeet]

On 29 November 2012 17:04, Frédéric Delanoy <frederic.delanoy at gmail.com> wrote:
>              case WINED3D_TOP_BUMPENVMAP_LUMINANCE:
>                  lum_map |= 1 << stage;
> +                /* fall through */
>              case WINED3D_TOP_BUMPENVMAP:
>                  bump_map |= 1 << stage;
> +                /* fall through */
>              case WINED3D_TOP_BLEND_TEXTURE_ALPHA:
>              case WINED3D_TOP_BLEND_TEXTURE_ALPHA_PM:
>                  tex_map |= 1 << stage;

I'm not opposed to these if they make it easier to find real bugs
inside the noise with Coverity, but I would also note that it should
be pretty obvious to anyone actually touching this code that the fall
through is intentional.