[d3d12] Dealing with D3D12 API quirks

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• D3D12CreateDevice() is a singleton per adapter. This makes it impossible to mix usage of non-debug followed by -debug d3d12 runtimes in the same application (at least not without a device_release happening between the transition). The other way around (-debug followed by non-debug) actually works, but the non-debug device will be the -debug device created previously...). To test these behaviors use lesson_12_using_the_gpu.cpp where there is a host_supports_target_device() call and a subsequent schedule_for_gpu(), each of which could be using a different target (or rather, a different -debug feature).