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

docs/news.md

@@ -14,6 +14,13 @@ values over 8. For compatibility with older versions, the
 @ref GLFW_UNLIMITED_MOUSE_BUTTONS input mode needs to be set to make use of
 this.
 
+
+### EGLConfig native access function {#eglconfig}
+
+GLFW now provides the @ref glfwGetEGLConfig native access function for querying
+the `EGLConfig` of a window that has a `EGLSurface`.
+
+
 ## Caveats {#caveats}
 
 ## Deprecations {#deprecations}
@@ -39,6 +46,9 @@ actively maintained and available on many platforms.
 
 ### New functions {#new_functions}
 
+ - @ref glfwGetEGLConfig
+
+
 ### New types {#new_types}
 
 ### New constants {#new_constants}