SDL: Enable transparent windows when using the D3D11 renderer

SDL Commits
1 
From 924de4df4865c325f905fca926490c7d4bf784e0 Mon Sep 17 00:00:00 2001
From: Sam Lantinga <[EMAIL REDACTED]>
Date: Mon, 13 Nov 2023 21:11:51 -0800
Subject: [PATCH] Enable transparent windows when using the D3D11 renderer

Note that this will not work with D3D12, as DXGI_SWAP_EFFECT_DISCARD is not available there.
---
 src/render/direct3d11/SDL_render_d3d11.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/src/render/direct3d11/SDL_render_d3d11.c b/src/render/direct3d11/SDL_render_d3d11.c
index 5e4eba0f40d0..2263f883276c 100644
--- a/src/render/direct3d11/SDL_render_d3d11.c
+++ b/src/render/direct3d11/SDL_render_d3d11.c
@@ -786,7 +786,12 @@ static HRESULT D3D11_CreateSwapChain(SDL_Renderer *renderer, int w, int h)
             swapChainDesc.Scaling = DXGI_SCALING_STRETCH;
         }
     }
-    swapChainDesc.SwapEffect = DXGI_SWAP_EFFECT_FLIP_SEQUENTIAL; /* All Windows Store apps must use this SwapEffect. */
+    if (SDL_GetWindowFlags(renderer->window) & SDL_WINDOW_TRANSPARENT) {
+        swapChainDesc.Scaling = DXGI_SCALING_STRETCH;
+        swapChainDesc.SwapEffect = DXGI_SWAP_EFFECT_DISCARD;
+    } else {
+        swapChainDesc.SwapEffect = DXGI_SWAP_EFFECT_FLIP_SEQUENTIAL; /* All Windows Store apps must use this SwapEffect. */
+    }
 #endif
     swapChainDesc.Flags = 0;