sdl12-compat: fixed build against SDL2 versions older than 2.0.14

SDL Commits
1 
From c5bd71de527b981ec736dbd94e973cc11de0cbc0 Mon Sep 17 00:00:00 2001
From: Ozkan Sezer <[EMAIL REDACTED]>
Date: Mon, 14 Jun 2021 14:01:20 +0300
Subject: [PATCH] fixed build against SDL2 versions older than 2.0.14

See: https://github.com/libsdl-org/sdl12-compat/issues/52#issuecomment-856782485
---
 src/SDL12_compat.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/src/SDL12_compat.c b/src/SDL12_compat.c
index c36de9d..a3c3d3a 100644
--- a/src/SDL12_compat.c
+++ b/src/SDL12_compat.c
@@ -3940,6 +3940,9 @@ SDL_SetVideoMode(int width, int height, int bpp, Uint32 flags12)
         bpp = SDL_BITSPERPIXEL(dmode.format);
     }
 
+    #if !SDL_VERSION_ATLEAST(2,0,14)
+    #define SDL_PIXELFORMAT_XRGB8888 SDL_PIXELFORMAT_RGB888
+    #endif
     switch (bpp) {
         case  8: appfmt = SDL_PIXELFORMAT_INDEX8; break;
         case 16: appfmt = SDL_PIXELFORMAT_RGB565; FIXME("bgr instead of rgb?"); break;