Fixed bug #562

this patch by Diego Pettenò <flameeyes@gentoo.org> for SDL-1.2:

The attached patch applies over latest ~arch SDL version, and allows to use the
xinerama support to launch an application full screened on the head "0".

The SDL_VIDEO_FULLSCREEN_HEAD environment variable sets the head on which the
full screen will be displayed, but if you set it to 0, the code simply ignores
it as unset. My patch changes the unset value to -1, so that the 0 value can be
used correctly. Without this, trying to get fullscreen on head 0 would get the
same result than not having xinerama enabled at all.

http://sources.gentoo.org/media-libs/libsdl/files/libsdl-1.2.11-xinerama-head-0.patch

SDL-1.3 doesnt seem to have any code like this, but it's still good for SDL-1.2

--HG--
branch : SDL-1.2

src/video/x11/SDL_x11modes.c

@@ -526,7 +526,7 @@ int X11_GetVideoModes(_THIS)
     /* Query Xinerama extention */
     if ( CheckXinerama(this, &xinerama_major, &xinerama_minor) ) {
         /* Find out which screen is the desired one */
-        int desired = 0;
+        int desired = -1;
         int screens;
         int w, h;
         SDL_NAME(XineramaScreenInfo) *xinerama;
@@ -647,7 +647,7 @@ int X11_GetVideoModes(_THIS)
     /* XVidMode */
     if ( !use_xrandr &&
 #if SDL_VIDEO_DRIVER_X11_XINERAMA
-         (!use_xinerama || xinerama_info.screen_number == 0) &&
+         (!use_xinerama || xinerama_info.screen_number == -1) &&
 #endif
          CheckVidMode(this, &vm_major, &vm_minor) &&
          SDL_NAME(XF86VidModeGetAllModeLines)(SDL_Display, SDL_Screen,&nmodes,&modes) )