Keep track of how many visuals we set up for GLcore, to avoid an inva…

…lid free.

The proper fix would involve actually setting up the ARGB visual for GLcore,
but I just want the server to not crash at exit.

GL/mesa/X/xf86glx.c

@@ -78,6 +78,7 @@ typedef struct __GLXMESAdrawable __GLXMESAdrawable;
 struct __GLXMESAscreen {
     __GLXscreen   base;
     int           index;
+    int		  num_vis;
     XMesaVisual  *xm_vis;
 };
 
@@ -280,7 +281,7 @@ __glXMesaScreenDestroy(__GLXscreen *screen)
     __GLXMESAscreen *mesaScreen = (__GLXMESAscreen *) screen;
     int i;
 
-    for (i = 0; i < screen->numVisuals; i++) {
+    for (i = 0; i < mesaScreen->num_vis; i++) {
 	if (mesaScreen->xm_vis[i])
 	    XMesaDestroyVisual(mesaScreen->xm_vis[i]);
     }
@@ -389,6 +390,7 @@ static void init_screen_visuals(__GLXMESAscreen *screen)
 
     xfree(used);
 
+    screen->num_vis = pScreen->numVisuals;
     screen->xm_vis = pXMesaVisual;
 }