demos: Fix vsync checking in glxgears

Of course glXGetVideoSyncSGI doesn't return the swap interval.  The feature
only exists in the Mesa extension... which is the whole reason I created the
Mesa extension!  Note that the Mesa extension allows drivers to default to a
swap interval of 0.  If the Mesa extension exists, use its value.  Only
consider the SGI extension when the Mesa extension is not available.

Fixes bug #22604.

progs/xdemos/glxgears.c

@@ -39,6 +39,12 @@
 #include <GL/gl.h>
 #include <GL/glx.h>
 
+#ifndef GLX_MESA_swap_control
+#define GLX_MESA_swap_control 1
+typedef int (*PFNGLXGETSWAPINTERVALMESAPROC)(void);
+#endif
+
+
 static int is_glx_extension_supported(Display *dpy, const char *query);
 
 static void query_vsync(Display *dpy);
@@ -592,31 +598,22 @@ query_vsync(Display *dpy)
    int interval = 0;
 
 
-#ifdef GLX_MESA_swap_control
-   if ((interval <= 0)
-       && is_glx_extension_supported(dpy, "GLX_MESA_swap_control")) {
+   if (is_glx_extension_supported(dpy, "GLX_MESA_swap_control")) {
       PFNGLXGETSWAPINTERVALMESAPROC pglXGetSwapIntervalMESA =
           (PFNGLXGETSWAPINTERVALMESAPROC)
           glXGetProcAddressARB((const GLubyte *) "glXGetSwapIntervalMESA");
 
       interval = (*pglXGetSwapIntervalMESA)();
+   } else if (is_glx_extension_supported(dpy, "GLX_SGI_swap_control")) {
+      /* The default swap interval with this extension is 1.  Assume that it
+       * is set to the default.
+       *
+       * Many Mesa-based drivers default to 0, but all of these drivers also
+       * export GLX_MESA_swap_control.  In that case, this branch will never
+       * be taken, and the correct result should be reported.
+       */
+      interval = 1;
    }
-#endif
-
-
-#ifdef GLX_SGI_video_sync
-   if ((interval <= 0)
-       && is_glx_extension_supported(dpy, "GLX_SGI_video_sync")) {
-      PFNGLXGETVIDEOSYNCSGIPROC pglXGetVideoSyncSGI =
-          (PFNGLXGETVIDEOSYNCSGIPROC)
-          glXGetProcAddressARB((const GLubyte *) "glXGetVideoSyncSGI");
-      unsigned count;
-
-      if ((*pglXGetVideoSyncSGI)(& count) == 0) {
-         interval = (int) count;
-      }
-   }
-#endif
 
 
    if (interval > 0) {