Backends: DX12: let the user specifies the DepthStencilView format.

[bmarques1995]

This is particullarly important for those who use RenderPasses

Let me explain the problem:

with this snippet:

`
m_CurrentBufferIndex = m_SwapChain->GetCurrentBackBufferIndex();
auto backBuffer = m_RenderTargets[m_CurrentBufferIndex];
auto rtvHandle = m_RTVHandles[m_CurrentBufferIndex];
auto dsvHandle = m_DSVHandle;

D3D12_RESOURCE_BARRIER rtSetupBarrier{};
rtSetupBarrier.Type = D3D12_RESOURCE_BARRIER_TYPE_TRANSITION;
rtSetupBarrier.Transition.pResource = backBuffer.Get();
rtSetupBarrier.Transition.StateBefore = D3D12_RESOURCE_STATE_PRESENT;
rtSetupBarrier.Transition.StateAfter = D3D12_RESOURCE_STATE_RENDER_TARGET;
rtSetupBarrier.Transition.Subresource = 0;
rtSetupBarrier.Flags = D3D12_RESOURCE_BARRIER_FLAG_NONE;

m_CommandLists[m_CurrentBufferIndex]->ResourceBarrier(1, &rtSetupBarrier);

D3D12_RENDER_PASS_RENDER_TARGET_DESC renderTargetDesc = {};
renderTargetDesc.cpuDescriptor = rtvHandle;
renderTargetDesc.BeginningAccess.Type = D3D12_RENDER_PASS_BEGINNING_ACCESS_TYPE_CLEAR;
renderTargetDesc.BeginningAccess.Clear.ClearValue = m_ClearColor;
renderTargetDesc.EndingAccess.Type = D3D12_RENDER_PASS_ENDING_ACCESS_TYPE_PRESERVE;

//D24_S8 dsv
D3D12_RENDER_PASS_DEPTH_STENCIL_DESC depthStencilDesc = {};
depthStencilDesc.cpuDescriptor = m_DSVHandle;
depthStencilDesc.DepthBeginningAccess.Type = D3D12_RENDER_PASS_BEGINNING_ACCESS_TYPE_CLEAR;
depthStencilDesc.DepthBeginningAccess.Clear.ClearValue.DepthStencil.Depth = 1.0f;
depthStencilDesc.DepthBeginningAccess.Clear.ClearValue.DepthStencil.Stencil = 0;
depthStencilDesc.DepthEndingAccess.Type = D3D12_RENDER_PASS_ENDING_ACCESS_TYPE_PRESERVE;

m_CommandLists[m_CurrentBufferIndex]->BeginRenderPass(1, &renderTargetDesc, &depthStencilDesc, D3D12_RENDER_PASS_FLAG_NONE);
`

and the previous version, the render was throwing:
D3D12 ERROR: ID3D12CommandList::DrawIndexedInstanced: The PSO indicates a format of DXGI_FORMAT_UNKNOWN to be bound to the rasterizer, but the render pass depth stencil descriptor indicates an incompatible format (DXGI_FORMAT_D24_UNORM_S8_UINT). [ EXECUTION ERROR #1170: RENDER_PASS_ERROR]

This snippet is to the use set also the DSV format to the Imgui PSO.

[ocornut]

Thank you Bruno for the PR and explanation.
I have merged this as 53dd755 with minor changes (added an explicit init_info.DSVFormat = DXGI_FORMAT_UNKNOWN; in the main.cpp for discoverability + changelog entries).