SDL: include: Add a comment about reserving a bit for sdl2-compat.

SDL Commits
1 
From 138f632b59f3a8f3089bb0e583420a0dab778f33 Mon Sep 17 00:00:00 2001
From: "Ryan C. Gordon" <[EMAIL REDACTED]>
Date: Thu, 29 Dec 2022 13:34:53 -0500
Subject: [PATCH] include: Add a comment about reserving a bit for sdl2-compat.

---
 include/SDL3/SDL_video.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/include/SDL3/SDL_video.h b/include/SDL3/SDL_video.h
index 2f43e1728289..d041cc56269b 100644
--- a/include/SDL3/SDL_video.h
+++ b/include/SDL3/SDL_video.h
@@ -103,6 +103,7 @@ typedef enum
 {
     SDL_WINDOW_FULLSCREEN = 0x00000001,         /**< fullscreen window */
     SDL_WINDOW_OPENGL = 0x00000002,             /**< window usable with OpenGL context */
+    /* 0x4 was SDL_WINDOW_SHOWN in SDL2, please reserve this bit for sdl2-compat. */
     SDL_WINDOW_HIDDEN = 0x00000008,             /**< window is not visible */
     SDL_WINDOW_BORDERLESS = 0x00000010,         /**< no window decoration */
     SDL_WINDOW_RESIZABLE = 0x00000020,          /**< window can be resized */