sdl2-compat: hint: Add SDL2_COMPAT hint for SDL3 compatibility features

SDL Commits
1 
From ec2781b66c9524541900cb7c8531a8c0e665fcc3 Mon Sep 17 00:00:00 2001
From: Cameron Gutman <[EMAIL REDACTED]>
Date: Thu, 30 Apr 2026 22:00:42 -0500
Subject: [PATCH] hint: Add SDL2_COMPAT hint for SDL3 compatibility features

---
 src/sdl2_compat.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/src/sdl2_compat.c b/src/sdl2_compat.c
index 3ba01349..2e282cbd 100644
--- a/src/sdl2_compat.c
+++ b/src/sdl2_compat.c
@@ -1087,6 +1087,7 @@ LoadSDL3(void)
                     }
                     SDL2Compat_ApplyQuirks(force_x11);  /* Apply and maybe print a list of any enabled quirks. */
                     SDL2Compat_SetEnvAtStartup("SDL3_VERSION", sdl3verstr);
+                    SDL2Compat_SetEnvAtStartup("SDL2_COMPAT", "1");
                 }
             }
             #define SDL3_SYM(rc,fn,params,args,ret) SDL3_##fn = (SDL3_##fn##_t) LoadSDL3Symbol("SDL_" #fn, &okay);