Fix DX12 menu crash on swapchains with more than 8 buffers

Size the menu's per-buffer arrays and the blur's descriptor slots for 16 back buffers, the DXGI maximum, refuse larger chains in CreateRenderTargetDx12 and skip frames with an out-of-range back buffer index.

OptiScaler/menu/menu_overlay_dx.cpp

@@ -15,7 +15,8 @@
 #include <shaders/menu_blur/MenuBlur_Dx12.h>
 
 // menu
-static int const NUM_BACK_BUFFERS = 8;
+// Sized for the largest swapchain DXGI allows, the menu can draw on chains created by other layers
+static int const NUM_BACK_BUFFERS = 16; // DXGI_MAX_SWAP_CHAIN_BUFFERS
 static int const SRV_HEAP_SIZE = 64;
 static bool _dx11Device = false;
 static bool _dx12Device = false;
@@ -101,6 +102,12 @@ static void CreateRenderTargetDx12(ID3D12Device* device, IDXGISwapChain* pSwapCh
         return;
     }
 
+    if (sd.BufferCount > NUM_BACK_BUFFERS)
+    {
+        LOG_ERROR("Swapchain has {} buffers, menu supports {}", sd.BufferCount, NUM_BACK_BUFFERS);
+        return;
+    }
+
     for (UINT i = 0; i < sd.BufferCount; ++i)
     {
         ID3D12Resource* pBackBuffer = nullptr;
@@ -500,10 +507,16 @@ static void RenderImGui_DX12(IDXGISwapChain* pSwapChainPlain)
         {
             _showRenderImGuiDebugOnce = true;
 
-            ImGui_ImplDX12_NewFrame();
-
             UINT backBufferIdx = pSwapChain->GetCurrentBackBufferIndex();
 
+            if (backBufferIdx >= NUM_BACK_BUFFERS || !g_mainRenderTargetResource[backBufferIdx])
+            {
+                pSwapChain->Release();
+                return;
+            }
+
+            ImGui_ImplDX12_NewFrame();
+
             // Blur textures are only created once the menu is opened, menu decides if they are used
             bool blurReady = false;
             if (g_menuBlur || MenuOverlayBase::IsVisible())

OptiScaler/shaders/menu_blur/MenuBlur_Dx12.h

@@ -10,7 +10,7 @@
 class MenuBlur_Dx12 : public Shader_Dx12, public MenuBlur_Common
 {
   private:
-    static constexpr UINT MaxBackBuffers = 8;
+    static constexpr UINT MaxBackBuffers = 16; // DXGI_MAX_SWAP_CHAIN_BUFFERS
 
     // Descriptor layout, every pass uses a SRV + UAV pair
     // [2 * i]: back buffer i SRV, [2 * i + 1]: blur A UAV