GEM has a window manager, of course

--HG--
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/trunk%402314

src/video/gem/SDL_gemvideo.c

@@ -479,6 +479,8 @@ GEM_VideoInit(_THIS, SDL_PixelFormat * vformat)
     SDL_AtariGL_InitPointers(this);
 #endif
 
+    this->info.wm_available = 1;
+
     /* We're done! */
     return (0);
 }