Add WgpuConfiguration::desired_maximum_frame_latency

[emilk]

Setting desired_maximum_frame_latency to a low value should theoretically lead to lower latency in winit apps using egui-wgpu (e.g. in eframe with wgpu backend).

• Replaces egui-wgpu: reduce input latency by defaulting to double-buffering #3714
• See also Expose maximum_frame_latency gfx-rs/wgpu#4899

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It seems like desired_maximum_frame_latency has no effect on my Mac. I lowered my monitor refresh-rate to 30Hz to test, and can see no difference between desired_maximum_frame_latency of 0 or 3.

Before when experimenting with changing the global DESIRED_NUM_FRAMES in wgpu I saw a huge difference, so I wonder what has changed.

I verified that set_maximum_drawable_count is being called with either 1 or 2, but I perceive no difference between the two.

[Wumpf]

what could possibly go wrong now 😄 🚢