Used glXGetProcAddressARB() when possible otherwise return NULL in glx.

It is more compatible than to use glXGetProcAddress() which is not always
present (need opengl >= 1.4).

modules/video_output/xcb/glx.c

@@ -470,7 +470,11 @@ static void SwapBuffers (vlc_gl_t *gl)
 static void *GetProcAddress (vlc_gl_t *gl, const char *name)
 {
     (void)gl;
-    return glXGetProcAddress ((const GLubyte *)name);
+#ifdef GLX_ARB_get_proc_address
+    return glXGetProcAddressARB ((const GLubyte *)name);
+#else
+    return NULL;
+#endif
 }
 
 /**