SDL: perf: fast-path pure palette animation for dos

SDL Commits
1 
From fed7f6fe06a4f216d8c28b9b7e4f278a7274514f Mon Sep 17 00:00:00 2001
From: palxex <[EMAIL REDACTED]>
Date: Thu, 9 Jul 2026 00:33:57 +0800
Subject: [PATCH] perf: fast-path pure palette animation for dos

---
 src/video/dos/SDL_dosframebuffer.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/src/video/dos/SDL_dosframebuffer.c b/src/video/dos/SDL_dosframebuffer.c
index 1500060ee580a..e77234110d211 100644
--- a/src/video/dos/SDL_dosframebuffer.c
+++ b/src/video/dos/SDL_dosframebuffer.c
@@ -502,6 +502,12 @@ bool DOSVESA_UpdateWindowFramebuffer(SDL_VideoDevice *device, SDL_Window *window
             }
         }
 
+        // No rectangles to update, so just program the palette and return.
+        // This is useful for pure palette animations.
+        if (numrects <= 0) {
+            return true;
+        }
+
         if (fb_state.use_dosmemput) {
             // dosmemput path (banked window, possibly multi-bank)
             const int src_pitch = fb_state.src_pitch;