Explicit narrowing cast from size_t to UINT (#2726)

Clang: `non-constant-expression cannot be narrowed from type 'size_t' (aka 'unsigned long long') to 'UINT' (aka 'unsigned int') in initializer list [-Wc++11-narrowing]`

examples/imgui_impl_dx12.cpp

@@ -502,9 +502,9 @@ bool    ImGui_ImplDX12_CreateDeviceObjects()
 
         // Create the input layout
         static D3D12_INPUT_ELEMENT_DESC local_layout[] = {
-            { "POSITION", 0, DXGI_FORMAT_R32G32_FLOAT,   0, IM_OFFSETOF(ImDrawVert, pos), D3D12_INPUT_CLASSIFICATION_PER_VERTEX_DATA, 0 },
-            { "TEXCOORD", 0, DXGI_FORMAT_R32G32_FLOAT,   0, IM_OFFSETOF(ImDrawVert, uv),  D3D12_INPUT_CLASSIFICATION_PER_VERTEX_DATA, 0 },
-            { "COLOR",    0, DXGI_FORMAT_R8G8B8A8_UNORM, 0, IM_OFFSETOF(ImDrawVert, col), D3D12_INPUT_CLASSIFICATION_PER_VERTEX_DATA, 0 },
+            { "POSITION", 0, DXGI_FORMAT_R32G32_FLOAT,   0, (UINT)IM_OFFSETOF(ImDrawVert, pos), D3D12_INPUT_CLASSIFICATION_PER_VERTEX_DATA, 0 },
+            { "TEXCOORD", 0, DXGI_FORMAT_R32G32_FLOAT,   0, (UINT)IM_OFFSETOF(ImDrawVert, uv),  D3D12_INPUT_CLASSIFICATION_PER_VERTEX_DATA, 0 },
+            { "COLOR",    0, DXGI_FORMAT_R8G8B8A8_UNORM, 0, (UINT)IM_OFFSETOF(ImDrawVert, col), D3D12_INPUT_CLASSIFICATION_PER_VERTEX_DATA, 0 },
         };
         psoDesc.InputLayout = { local_layout, 3 };
     }