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

docs/news.md

@@ -21,6 +21,12 @@ GLFW now provides the @ref glfwGetEGLConfig native access function for querying
 the `EGLConfig` of a window that has a `EGLSurface`.
 
 
+### GLXFBConfig native access function {#glxfbconfig}
+
+GLFW now provides the @ref glfwGetGLXFBConfig native access function for
+querying the `GLXFBConfig` of a window that has a `GLXWindow`.
+
+
 ## Caveats {#caveats}
 
 ## Deprecations {#deprecations}
@@ -47,6 +53,7 @@ actively maintained and available on many platforms.
 ### New functions {#new_functions}
 
  - @ref glfwGetEGLConfig
+ - @ref glfwGetGLXFBConfig
 
 
 ### New types {#new_types}