Render R2: persist named render setups without re-evaluating geometry (#667)

render.settings.set saves validated named render setups (camera, output size, exposure, environment) as one undo step, and render.settings.get reads them back without applying a camera. When validated render settings are the only change, the engine keeps the evaluated model instead of rebuilding it. Setups are saved under design format 21; format-20 designs read as having none. Fixes #658; refs #620.

Contributed by @0xlillith.

Co-authored-by: lillith <[email protected]>

crates/doc/src/document.rs

@@ -1614,6 +1614,9 @@ pub struct DesignLink {
 /// The document.
 #[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
 pub struct Document {
+    /// Named, renderer-independent setups. Device/job state is never saved here.
+    #[serde(default, skip_serializing_if = "crate::render_settings::RenderSettings::is_empty")]
+    pub render: std::sync::Arc<crate::render_settings::RenderSettings>,
     #[serde(default = "default_format")]
     pub format: String,
     pub name: String,
@@ -1828,6 +1831,7 @@ impl Default for Document {
 impl Document {
     pub fn new(name: &str) -> Self {
         Document {
+            render: Default::default(),
             format: default_format(),
             name: name.into(),
             units: default_units(),
@@ -1859,6 +1863,18 @@ impl Document {
         }
     }
 
+    /// A complete structural proof: no non-render field is allowed to differ. Keeping this
+    /// comparison based on the whole Document also includes fields added in future versions.
+    /// The clone/comparison is only needed when render intent actually differs.
+    pub fn differs_only_in_render(&self, previous: &Document) -> bool {
+        if std::sync::Arc::ptr_eq(&self.render, &previous.render) || self.render == previous.render || self.render.validate().is_err() {
+            return false;
+        }
+        let mut normalized = previous.clone();
+        normalized.render = self.render.clone();
+        *self == normalized
+    }
+
     /// The colour a body is shown and exported in (sRGB as 0–1 fractions): its appearance,
     /// else its component's, else its material's look, else the colour it was imported with.
     /// (Faces may have their own: `face_looks`.)