Add a glfwGetGLXFBConfig() function.

This is needed with OpenXR, to create the binding with the graphics
platform.

An issue is open about that need:
https://github.com/glfw/glfw/issues/2129

include/GLFW/glfw3native.h

@@ -478,6 +478,21 @@ GLFWAPI GLXContext glfwGetGLXContext(GLFWwindow* window);
  *  @ingroup native
  */
 GLFWAPI GLXWindow glfwGetGLXWindow(GLFWwindow* window);
+
+/*! @brief Returns the `GLXFBConfig` that was chosen to create the
+ *  specified window.
+ *
+ *  @return The `GLXFBConfig` that was chosen to create the specified
+ *  window, or `NULL` if an [error](@ref error_handling) occurred.
+ *
+ *  @thread_safety This function may be called from any thread.  Access is not
+ *  synchronized.
+ *
+ *  @since Added in version 3.4
+ *
+ *  @ingroup native
+ */
+GLFWAPI GLXFBConfig glfwGetGLXFBConfig(GLFWwindow* window);
 #endif
 
 #if defined(GLFW_EXPOSE_NATIVE_WAYLAND)

src/glx_context.c

@@ -246,6 +246,11 @@ static void destroyContextGLX(_GLFWwindow* window)
         glXDestroyContext(_glfw.x11.display, window->context.glx.handle);
         window->context.glx.handle = NULL;
     }
+
+    if (window->context.glx.fbconfig)
+    {
+        window->context.glx.fbconfig = NULL;
+    }
 }
 
 
@@ -788,5 +793,18 @@ GLFWAPI GLXWindow glfwGetGLXWindow(GLFWwindow* handle)
     return window->context.glx.window;
 }
 
-#endif // _GLFW_X11
+GLFWAPI GLXFBConfig glfwGetGLXFBConfig(GLFWwindow* handle)
+{
+    _GLFWwindow* window = (_GLFWwindow*) handle;
+    _GLFW_REQUIRE_INIT_OR_RETURN(None);
 
+    if (window->context.client == GLFW_NO_API)
+    {
+        _glfwInputError(GLFW_NO_WINDOW_CONTEXT, NULL);
+        return NULL;
+    }
+
+    return window->context.glx.fbconfig;
+}
+
+#endif // _GLFW_X11