winex11: Add a few 'fall through' comments.

[Huw Davies]

On Tue, Nov 27, 2012 at 06:44:35PM +0800, Dmitry Timoshkov wrote:
>     case DirectColor:
> 	X11DRV_PALETTE_PaletteFlags |= X11DRV_PALETTE_VIRTUAL;
>         /* fall through */
>     case GrayScale:
> 
> would be more clear IMHO. Right now it's not obvious to which case statement
> 'fall through' belongs to.

Resent, thanks.

Huw.