wgpu: Allow configuring VSync and frame latency at runtime (#8114)

Let apps change present_mode and desired_maximum_frame_latency at
runtime instead of only at startup.
API changes (egui-wgpu):
- New SurfaceConfig { present_mode, desired_maximum_frame_latency }.
- WgpuConfiguration now nests these as pub surface: SurfaceConfig (was
two top-level fields).
- RenderState gains pub surface_config: SurfaceConfig — the
currently-requested value.
API additions (eframe):
- Frame::wgpu_surface_config() / Frame::set_wgpu_surface_config(...) for
get/set.
- SurfaceConfig re-exported as eframe::SurfaceConfig.
                                                       
How it works:
The wgpu painter compares render_state.surface_config to its
currently-applied values each paint. If they differ it updates its
config and flips
needs_reconfigure on every surface, piggybacking on the existing
deferred-reconfigure pathway.
                                                       
Demo:
The backend panel (egui_demo_app) gets dropdowns for present mode and
desired max frame latency, wired through the new Frame accessors.

<img width="282" height="172" alt="image"
src="https://github.com/user-attachments/assets/0b1274b2-7e4e-4413-969b-0a014c415f79"
/>

crates/eframe/src/epi.rs

@@ -469,7 +469,8 @@ impl Default for NativeOptions {
             centered: false,
 
             #[cfg(feature = "wgpu_no_default_features")]
-            wgpu_options: egui_wgpu::WgpuConfiguration::default(),
+            wgpu_options: egui_wgpu::WgpuConfiguration::default()
+                .with_surface_config(egui_wgpu::SurfaceConfig::LOW_LATENCY),
 
             persist_window: true,
 
@@ -793,6 +794,28 @@ impl Frame {
     pub fn wgpu_render_state(&self) -> Option<&egui_wgpu::RenderState> {
         self.wgpu_render_state.as_ref()
     }
+
+    /// The currently-applied runtime surface config (present mode, frame latency)
+    /// used by the `wgpu` renderer, if any.
+    ///
+    /// Returns `None` when not using the `wgpu` backend.
+    #[cfg(feature = "wgpu_no_default_features")]
+    pub fn wgpu_surface_config(&self) -> Option<egui_wgpu::SurfaceConfig> {
+        self.wgpu_render_state
+            .as_ref()
+            .map(|state| state.surface_config)
+    }
+
+    /// Set the runtime surface config (present mode, frame latency) for the `wgpu`
+    /// renderer. The surface is reconfigured on the next paint.
+    ///
+    /// No-op when not using the `wgpu` backend.
+    #[cfg(feature = "wgpu_no_default_features")]
+    pub fn set_wgpu_surface_config(&mut self, config: egui_wgpu::SurfaceConfig) {
+        if let Some(state) = &mut self.wgpu_render_state {
+            state.surface_config = config;
+        }
+    }
 }
 
 /// Information about the web environment (if applicable).

crates/eframe/src/lib.rs

@@ -159,7 +159,7 @@ pub use {egui, egui::emath, egui::epaint};
 pub use {egui_glow, glow};
 
 #[cfg(feature = "wgpu_no_default_features")]
-pub use {egui_wgpu, egui_wgpu::wgpu};
+pub use {egui_wgpu, egui_wgpu::SurfaceConfig, egui_wgpu::WgpuConfiguration, egui_wgpu::wgpu};
 
 mod epi;
 

crates/eframe/src/web/web_painter_wgpu.rs

@@ -118,7 +118,7 @@ impl WebPainterWgpu {
 
         let surface_configuration = wgpu::SurfaceConfiguration {
             format: render_state.target_format,
-            present_mode: wgpu_options.present_mode,
+            present_mode: wgpu_options.surface.present_mode,
             view_formats: vec![render_state.target_format],
             ..default_configuration
         };