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/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
         };