Don't duplicate extension preference logic.

src/x11_window.c

@@ -599,8 +599,6 @@ static void initGLXExtensions(_GLFWwindow* window)
             window->GLX.EXT_swap_control = GL_TRUE;
     }
 
-    if (!window->GLX.EXT_swap_control)
-    {
     if (_glfwPlatformExtensionSupported("GLX_SGI_swap_control"))
     {
         window->GLX.SwapIntervalSGI = (PFNGLXSWAPINTERVALSGIPROC)
@@ -609,7 +607,6 @@ static void initGLXExtensions(_GLFWwindow* window)
         if (window->GLX.SwapIntervalSGI)
             window->GLX.SGI_swap_control = GL_TRUE;
     }
-    }
 
     if (_glfwPlatformExtensionSupported("GLX_SGIX_fbconfig"))
     {
@@ -643,26 +640,16 @@ static void initGLXExtensions(_GLFWwindow* window)
             window->GLX.ARB_create_context = GL_TRUE;
     }
 
-    if (window->GLX.ARB_create_context)
-    {
+    if (_glfwPlatformExtensionSupported("GLX_ARB_create_context_robustness"))
+        window->GLX.ARB_create_context_robustness = GL_TRUE;
+
     if (_glfwPlatformExtensionSupported("GLX_ARB_create_context_profile"))
         window->GLX.ARB_create_context_profile = GL_TRUE;
-    }
 
-    if (window->GLX.ARB_create_context &&
-        window->GLX.ARB_create_context_profile)
-    {
     if (_glfwPlatformExtensionSupported("GLX_EXT_create_context_es2_profile"))
         window->GLX.EXT_create_context_es2_profile = GL_TRUE;
 }
 
-    if (window->GLX.ARB_create_context)
-    {
-        if (_glfwPlatformExtensionSupported("GLX_ARB_create_context_robustness"))
-            window->GLX.ARB_create_context_robustness = GL_TRUE;
-    }
-}
-
 
 //========================================================================
 // Create the X11 window (and its colormap)