SDL: include: add comment documenting the change(s) to SDL_opengl_glext.h (c5035)

SDL Commits
1 
From c5035d8ffdf45a2bbadf81a7095fa681009f8149 Mon Sep 17 00:00:00 2001
From: Anonymous Maarten <[EMAIL REDACTED]>
Date: Sat, 4 Feb 2023 02:44:10 +0100
Subject: [PATCH] include: add comment documenting the change(s) to
 SDL_opengl_glext.h

---
 include/SDL_opengl_glext.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/include/SDL_opengl_glext.h b/include/SDL_opengl_glext.h
index bdd3efb79a6a..f5da65d8f230 100644
--- a/include/SDL_opengl_glext.h
+++ b/include/SDL_opengl_glext.h
@@ -1,3 +1,6 @@
+// SDL modified the include guard to be compatible with Mesa and Apple include guards:
+// - Mesa uses: __gl_glext_h_
+// - Apple uses: __glext_h_
 #if !defined(__glext_h_) && !defined(__gl_glext_h_)
 #define __glext_h_ 1
 #define __gl_glext_h_ 1