SDL: gpu: Fix swapchain texture cycling.

SDL Commits
1 
From 96e05da95e5e4636af64fa3f7d7c924967201c35 Mon Sep 17 00:00:00 2001
From: Ethan Lee <[EMAIL REDACTED]>
Date: Sat, 7 Mar 2026 10:36:19 -0500
Subject: [PATCH] gpu: Fix swapchain texture cycling.

This is actually a 1:1 copy of a patch we wrote for GPU PS5 some time ago!
---
 src/gpu/d3d12/SDL_gpu_d3d12.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/src/gpu/d3d12/SDL_gpu_d3d12.c b/src/gpu/d3d12/SDL_gpu_d3d12.c
index 5a1018a550a36..d21a998ac307b 100644
--- a/src/gpu/d3d12/SDL_gpu_d3d12.c
+++ b/src/gpu/d3d12/SDL_gpu_d3d12.c
@@ -8061,7 +8061,10 @@ static bool D3D12_Submit(
 
         windowData->inFlightFences[windowData->frameCounter] = (SDL_GPUFence *)d3d12CommandBuffer->inFlightFence;
         (void)SDL_AtomicIncRef(&d3d12CommandBuffer->inFlightFence->referenceCount);
-        windowData->frameCounter = (windowData->frameCounter + 1) % renderer->allowedFramesInFlight;
+
+        // Normally this is '% allowedFramesInFlight', but the value gets clamped
+        // at swapchain creation time, so use swapchainTextureCount instead
+        windowData->frameCounter = (windowData->frameCounter + 1) % windowData->swapchainTextureCount;
     }
 
     // Check for cleanups