Add glfwGetGLXFBConfig native access function

This adds the glfwGetGLXFBConfig function for querying the GLXFBConfig
the GLXWindow of a window.

This commit is a squashed and modified version of PR #1925 by knokko.
The following changes were made by elmindreda:

The function signature was changed to handle GLXFBConfig being an opaque
value in core GLX.  The function error checks were fixed and updated.
The struct member name was changed.  The struct member clearing on
context destruction was removed.  All documentation snippets were
updated.

Closes #1925

include/GLFW/glfw3native.h

@@ -478,6 +478,29 @@ GLFWAPI GLXContext glfwGetGLXContext(GLFWwindow* window);
  *  @ingroup native
  */
 GLFWAPI GLXWindow glfwGetGLXWindow(GLFWwindow* window);
+
+/*! @brief Retrieves the `GLXFBConfig` of the specified window's `GLXWindow`.
+ *
+ *  @param[in] window The window whose `GLXWindow` to query.
+ *  @param[out] config The `GLXFBConfig` of the window `GLXWindow`, if available.
+ *  @return `GLFW_TRUE` if successful, or `GLFW_FALSE` if an
+ *  [error](@ref error_handling) occurred.
+ *
+ *  @errors Possible errors include @ref GLFW_NOT_INITIALIZED, @ref
+ *  GLFW_NO_WINDOW_CONTEXT and @ref GLFW_PLATFORM_UNAVAILABLE.
+ *
+ *  @remark `GLXFBConfig` is an opaque type.  Unlike other GLFW functions, the
+ *  @p config out parameter is not cleared on error, as core GLX does not define
+ *  any invalid value.
+ *
+ *  @thread_safety This function may be called from any thread.  Access is not
+ *  synchronized.
+ *
+ *  @since Added in version 3.5
+ *
+ *  @ingroup native
+ */
+GLFWAPI int glfwGetGLXFBConfig(GLFWwindow* window, GLXFBConfig* config);
 #endif
 
 #if defined(GLFW_EXPOSE_NATIVE_WAYLAND)