Add glfwGetEGLConfig native access function

This adds the glfwGetEGLConfig function for querying the EGLConfig of
the EGLSurface of a window.

This is a re-implementation of the PR #2045 by knokko, slightly
redesigned to handle EGLConfig being an opaque type in core EGL.

Closes #2045

include/GLFW/glfw3native.h

@@ -586,6 +586,29 @@ GLFWAPI EGLContext glfwGetEGLContext(GLFWwindow* window);
  *  @ingroup native
  */
 GLFWAPI EGLSurface glfwGetEGLSurface(GLFWwindow* window);
+
+/*! @brief Retrieves the `EGLConfig` of the specified window's `EGLSurface`.
+ *
+ *  @param[in] window The window whose `EGLSurface` to query.
+ *  @param[out] config The `EGLConfig` of the window `EGLSurface`, 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 and @ref
+ *  GLFW_NO_WINDOW_CONTEXT.
+ *
+ *  @remark `EGLConfig` is an opaque type.  Unlike other GLFW functions, the @p
+ *  config out parameter is not cleared on error, as core EGL 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 glfwGetEGLConfig(GLFWwindow* window, EGLConfig* config);
 #endif
 
 #if defined(GLFW_EXPOSE_NATIVE_OSMESA)