SDL: testsprite: only use transparent alpha on transparent windows

SDL Commits
1 
From d92079f2b78edf4d17420893fc0e8d77d6a5b5a8 Mon Sep 17 00:00:00 2001
From: Sam Lantinga <[EMAIL REDACTED]>
Date: Thu, 9 Oct 2025 14:32:08 -0700
Subject: [PATCH] testsprite: only use transparent alpha on transparent windows

Fixes https://github.com/libsdl-org/SDL/issues/14105
---
 test/testsprite.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/test/testsprite.c b/test/testsprite.c
index b4bfcaa259ee4..cd85299e7fe61 100644
--- a/test/testsprite.c
+++ b/test/testsprite.c
@@ -119,7 +119,11 @@ static void MoveSprites(SDL_Renderer *renderer, SDL_Texture *sprite)
     }
 
     /* Draw a gray background */
-    SDL_SetRenderDrawColor(renderer, 0xA0, 0xA0, 0xA0, 0x00 /* used with --transparent */);
+    if (SDL_GetWindowFlags(SDL_GetRenderWindow(renderer)) & SDL_WINDOW_TRANSPARENT) {
+        SDL_SetRenderDrawColor(renderer, 0xA0, 0xA0, 0xA0, SDL_ALPHA_TRANSPARENT);
+    } else {
+        SDL_SetRenderDrawColor(renderer, 0xA0, 0xA0, 0xA0, SDL_ALPHA_OPAQUE);
+    }
     SDL_RenderClear(renderer);
 
     /* Test points */