Allow disabling waiting for latency waitable object

[marcpabst]

This PR allows you to skip the frame latency waitable mechanism used for the DX12 backend by adding a new option to Dx12BackendOptions. The options now also carry over to the DX12 Surface, and a waitable object is only obtained when use_latency_waitable_object == true. The swap chain still gets created with the DXGI_SWAP_CHAIN_FLAG_FRAME_LATENCY_WAITABLE_OBJECT flag, because I don't think there are any downsides and it lets users still obtain a waitable object themselves.

Not sure if this is the best way to do this or the right place for this setting, so let me know what you think. Happy to also change the docs.

[cwfitzgerald]

Unless I missed something, seems to be missing the part where None doesn't make a swapchain with waitable object support.

Code looks pretty darn good otherwise :)

[cwfitzgerald]

This feels weird being pub

[marcpabst]

My idea was that you don't need to expose the actual waitable handle this way. So wgpu can still handle the configuration of the swap chain and creation of the waitable object but we can abstract away the actual unsafe syscalls.

[marcpabst]

Unless I missed something, seems to be missing the part where None doesn't make a swapchain with waitable object support.

Code looks pretty darn good otherwise :)

Yes, I thought that's the best way to deal with this. There's no documented downside to setting this flag, but with my (very limited) DX12 experience, that's not to say there are no side effects at all. So it might be better to at least give users an option to disable it.