SDL: Added SDL_FLIP_DIAGONAL

From b9ab3269823ea329757aada3a20ef97abf1a9f6d Mon Sep 17 00:00:00 2001
From: RPP-dev <[EMAIL REDACTED]>
Date: Fri, 14 Jul 2023 13:24:15 +0200
Subject: [PATCH] Added SDL_FLIP_DIAGONAL

Since SDL_RenderFlip is an enum, SDL_FLIP_HORIZONTAL and SDL_FLIP_VERTICAL can not be OR'ed to get the "SDL_FLIP_DIAGONAL".
Render code is actually able to perform these 3 kind of "flipping" so I just added a new enum called SDL_FLIP_DIAGONAL with the OR'ed value (3) so it can be used.
---
 include/SDL3/SDL_surface.h | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/include/SDL3/SDL_surface.h b/include/SDL3/SDL_surface.h
index d42604fad69e..aac7f4548f69 100644
--- a/include/SDL3/SDL_surface.h
+++ b/include/SDL3/SDL_surface.h
@@ -80,9 +80,10 @@ typedef enum
  */
 typedef enum
 {
-    SDL_FLIP_NONE,          /**< Do not flip */
-    SDL_FLIP_HORIZONTAL,    /**< flip horizontally */
-    SDL_FLIP_VERTICAL       /**< flip vertically */
+    SDL_FLIP_NONE       = 0x00,     /**< Do not flip */
+    SDL_FLIP_HORIZONTAL = 0x01,     /**< flip horizontally */
+    SDL_FLIP_VERTICAL   = 0x02,     /**< flip vertically */
+    SDL_FLIP_DIAGONAL   = 0x03,     /**< flip diagonally (both horizontally and vertically) */
 } SDL_FlipMode;
 
 /**