Fix #306: render on the power-saving GPU by default, with a preference for the discrete one

On hybrid-graphics laptops (an integrated GPU driving the display plus a
discrete one), egui-wgpu's default power preference is HighPerformance, so
VectorCraft rendered on the discrete GPU and its frames were copied to the
display through the integrated one. On an Asus ZenBook with Intel Iris + Arc
the window flickered whenever the mouse moved over it (#306, #305).

The canvas is rasterized on the CPU (vello_cpu) and the GPU only composites
the canvas texture and draws the UI, so the integrated GPU is plenty. The
desktop app now asks wgpu for the power-saving adapter by default.
Single-GPU machines are unaffected.

- Preferences > Performance > Graphics Processor (`gpuPreference`:
  powerSaving, the default, or highPerformance), with a note that it
  applies after a restart. The app reads it from the saved preferences
  before the window opens. WGPU_POWER_PREF still overrides it.
- Help > About and `ui.inspect` (`graphicsAdapter`) name the adapter in
  use, and it is logged at startup, for future GPU bug reports.
- Docs (README, docs/development.md) and ROADMAP updated.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/vectorcraft/src/main.rs

@@ -241,6 +241,27 @@ fn app_icon() -> egui::IconData {
     eframe::icon_data::from_png_bytes(png).unwrap_or_default()
 }
 
+/// The power preference wgpu picks the window's graphics adapter with (#306): the
+/// `WGPU_POWER_PREF` environment variable (`low`, `high`, `none`) when set, otherwise Preferences ›
+/// Performance › Graphics Processor (`gpuPreference`). Power saving unless the preference asks for
+/// high performance: the canvas is rasterized on the CPU and the GPU only composites it, which the
+/// integrated GPU of a hybrid-graphics laptop does easily, while presenting frames rendered on the
+/// discrete GPU through the integrated one made the window flicker on some laptops. With a single
+/// GPU both preferences pick it.
+fn power_preference(pref: Option<&str>, env: Option<eframe::wgpu::PowerPreference>) -> eframe::wgpu::PowerPreference {
+    use eframe::wgpu::PowerPreference;
+    match (env, pref) {
+        (Some(p), _) => p,
+        (None, Some("highPerformance")) => PowerPreference::HighPerformance,
+        (None, _) => PowerPreference::LowPower,
+    }
+}
+
+/// "name (backend, kind)" of the adapter the window renders with, for Help › About and bug reports.
+fn adapter_summary(info: &eframe::wgpu::AdapterInfo) -> String {
+    format!("{} ({:?}, {:?})", info.name.trim(), info.backend, info.device_type)
+}
+
 /// Windows and Linux: no OS title bar; the app bar is the title bar (`vectorcraft_ui_egui::titlebar`).
 /// macOS keeps its traffic lights over the integrated title strip.
 const CUSTOM_TITLEBAR: bool = !cfg!(target_os = "macos");
@@ -262,7 +283,9 @@ fn main() -> eframe::Result {
     }
     let saved = read_prefs();
     let saved_window = saved.as_ref().and_then(|ui| ui.window);
-    let options = eframe::NativeOptions {
+    let gpu_pref = saved.as_ref().and_then(|ui| ui.engine_prefs.get("gpuPreference")).and_then(serde_json::Value::as_str);
+    let power = power_preference(gpu_pref, eframe::wgpu::PowerPreference::from_env());
+    let mut options = eframe::NativeOptions {
         viewport: egui::ViewportBuilder::default()
             .with_title("VectorCraft")
             .with_inner_size(window::DEFAULT_SIZE)
@@ -276,6 +299,9 @@ fn main() -> eframe::Result {
             .with_app_id("ai.storyteller.vectorcraft"),
         ..Default::default()
     };
+    if let eframe::egui_wgpu::WgpuSetup::CreateNew(create) = &mut options.wgpu_options.wgpu_setup {
+        create.power_preference = power;
+    }
     eframe::run_native(
         "VectorCraft",
         options,
@@ -297,6 +323,11 @@ fn main() -> eframe::Result {
                 let recovery = prefs_path().and_then(|p| Some(p.parent()?.join("Data Recovery").to_string_lossy().to_string()));
                 app.session.recovery.set_default_folder(recovery);
             }
+            if let Some(rs) = &cc.wgpu_render_state {
+                let summary = adapter_summary(&rs.adapter.get_info());
+                log::info!("vectorcraft: rendering with {summary} (power preference {power:?})");
+                app.graphics_adapter = Some(summary);
+            }
             app.integrated_titlebar = cfg!(target_os = "macos");
             app.custom_titlebar = CUSTOM_TITLEBAR;
             if let Some(port) = control_port {
@@ -316,3 +347,35 @@ fn main() -> eframe::Result {
         }),
     )
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use eframe::wgpu::PowerPreference;
+
+    #[test]
+    fn power_saving_unless_the_preference_or_the_environment_says_otherwise() {
+        assert_eq!(power_preference(None, None), PowerPreference::LowPower);
+        assert_eq!(power_preference(Some("powerSaving"), None), PowerPreference::LowPower);
+        assert_eq!(power_preference(Some("highPerformance"), None), PowerPreference::HighPerformance);
+        // A value this version doesn't know (a newer or damaged preference file) is the default.
+        assert_eq!(power_preference(Some("turbo"), None), PowerPreference::LowPower);
+        assert_eq!(power_preference(Some(""), None), PowerPreference::LowPower);
+        // WGPU_POWER_PREF wins over the preference, either way.
+        assert_eq!(power_preference(Some("powerSaving"), Some(PowerPreference::HighPerformance)), PowerPreference::HighPerformance);
+        assert_eq!(power_preference(Some("highPerformance"), Some(PowerPreference::LowPower)), PowerPreference::LowPower);
+        assert_eq!(power_preference(None, Some(PowerPreference::None)), PowerPreference::None);
+    }
+
+    /// The preference the engine saves is the one the app reads back before the window opens.
+    #[test]
+    fn the_saved_engine_preference_is_found() {
+        let mut prefs = vectorcraft_engine::Prefs::default();
+        let saved = prefs.to_json();
+        assert_eq!(saved.get("gpuPreference").and_then(serde_json::Value::as_str), Some("powerSaving"));
+        prefs.gpu_preference = "highPerformance".into();
+        let saved = prefs.to_json();
+        let pref = saved.get("gpuPreference").and_then(serde_json::Value::as_str);
+        assert_eq!(power_preference(pref, None), PowerPreference::HighPerformance);
+    }
+}