Present iris with vsync

AutoNoVsync accepts frames as fast as the GPU will take them, so a redraw
burst costs whatever the hardware can be made to do rather than one frame.
That is the wrong default for a toolkit whose stated goal is to save
battery, and it is worse than wrong on this machine: the GPU here is the
host's real one reached through virtio-gpu, so frames nobody will see are
paid for on somebody's desktop.

AutoVsync picks Fifo, which every backend supports.

Note this is not an idle drain -- iris only draws when needs_redraw says
something changed, and the tabs example guards its stats string -- so this
bounds the cost of a burst rather than stopping a spin.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

iris/src/default/render.rs

@@ -115,7 +115,14 @@ impl UiRenderer {
             format: surface_format,
             width: size.width,
             height: size.height,
-            present_mode: PresentMode::AutoNoVsync,
+            // Vsync, because a UI toolkit aiming at battery life must not
+            // present frames a display will never show. AutoNoVsync accepts
+            // them as fast as the GPU will take them, so a redraw burst costs
+            // whatever the hardware can be made to do rather than one frame --
+            // and on this machine the GPU is the host's real one, reached
+            // through virtio-gpu, so that cost lands on somebody's desktop.
+            // AutoVsync picks Fifo, which every backend supports.
+            present_mode: PresentMode::AutoVsync,
             alpha_mode: surface_caps.alpha_modes[0],
             desired_maximum_frame_latency: 2,
             view_formats: vec![],