Improve input latency by switching to double-buffered rendering

This makes a huge difference in perceived latency on my 60 Hz monitor
on macOS on a native build, e.g. when hovering buttons,
or just the distance between the cursor and the hover-window
when hovering an image.

It does produce some tearing, noticable e.g. when quickly panning
a 2D image. To me the trade-off is absolutely worth it:
tearing is a mild visual glitch that shows up occationally,
input latency is a constant irritation, making an application feel
sluggish.

It makes no difference on web though, so this does NOT
solve #2672

crates/re_viewer/src/lib.rs

@@ -154,6 +154,10 @@ pub(crate) fn wgpu_options() -> egui_wgpu::WgpuConfiguration {
             }),
             supported_backends: re_renderer::config::supported_backends(),
             device_descriptor: std::sync::Arc::new(|adapter| re_renderer::config::DeviceCaps::from_adapter(adapter).device_descriptor()),
+
+            present_mode: wgpu::PresentMode::AutoNoVsync, // double-buffered: low-latency, may have tearing
+            // present_mode: wgpu::PresentMode::AutoVsync, // triple-buffered: high-latency, no tearing
+
             ..Default::default()
         }
 }