Clarify swapchain image initial EB layout

[jeremyong]

I believe it's clear that swapchain images are retrieved in the D3D12_BARRIER_LAYOUT_COMMON layout, given that with legacy resources, I believe they started in the equivalently mapped D3D12_RESOURCE_STATE_COMMON, but for engineers learning DX12 starting from enhanced barriers in the future without legacy resource state experience, it may be worth mentioning swapchain images in the EB spec for completeness. Thanks!

[billkris-ms]

Thank you @jeremyong. This is a good suggestion. I'll update the spec shortly.

[jeremyong]

Thanks! The change looks good. On the subject of presentation and the swapchain, I suppose there is perhaps some implicit synchronization happening after command list execution and IDXGISwapchain::Present also? It's something I've been used to with DirectX APIs, but I'll note the lack of any explicit barrier needed prior to presentation (compare to Vulkan where there is this notion of an "external" dependency).