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

@@ -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;