Backends: DX12: Added extra ID3D12DescriptorHeap parameter to ImGui_ImplDX12_Init() function. The value is unused in master branch but will be used by the multi-viewport features (docking branch). (#2851)

+ Using SafeRelease() in master.

examples/example_win32_directx12/main.cpp

@@ -85,7 +85,7 @@ int main(int, char**)
     // Setup Platform/Renderer bindings
     ImGui_ImplWin32_Init(hwnd);
     ImGui_ImplDX12_Init(g_pd3dDevice, NUM_FRAMES_IN_FLIGHT,
-        DXGI_FORMAT_R8G8B8A8_UNORM,
+        DXGI_FORMAT_R8G8B8A8_UNORM, g_pd3dSrvDescHeap,
         g_pd3dSrvDescHeap->GetCPUDescriptorHandleForHeapStart(),
         g_pd3dSrvDescHeap->GetGPUDescriptorHandleForHeapStart());