Export: Save for Office, CSS and Copy warn when an image couldn't be decoded or a font is missing

Save for Office Documents, the pictures that css.export rasterizes and
the PNG and PDF that Copy offers other apps were encoded with
fileio::encode, which drops the encoder's warnings. When the art held
an image that the renderer could not decode, the PNGs lacked it; type
in a font that is not installed came out in the fallback font in the
Office PNG and Copy's PNG and PDF. No reply or status message said so.
The app also dropped the warnings that document.exportSelection
returns.

document.exportForOffice, clipboard.exportPng and clipboard.exportPdf
now return `warnings`, and css.export adds the notes of its pictures to
its `warnings`, each once. Save for Office Documents and
document.exportSelection show the first note in the status bar in Save
As's message, and the CSS Properties panel's Export Selected CSS… and
Export All… show it in Export As's message. After Copy or Cut, once
the PNG and PDF are on the system clipboard, the status bar shows their
first note. The files are written as before.

Docs: docs/mcp.md (Clipboard, CSS Properties), docs/control-protocol.md.

crates/engine/src/cmd/css.rs

@@ -7,7 +7,7 @@ use serde_json::{Value, json};
 use vectorcraft_doc::NodeId;
 use vectorcraft_svg::{CssOptions, CssSheet, css_rules};
 
-use super::fileio::{default_name, encode, isolated, write_file};
+use super::fileio::{default_name, encode_with_warnings, isolated, write_file};
 use super::*;
 
 pub fn specs() -> Vec<CommandSpec> {
@@ -27,7 +27,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "Export CSS",
             [],
             None,
-            "{path?, scope?: selection|all (default selection; ids? picks the objects), …options as css.selection, id?: object id (default: the selection)} write the CSS (css.selection, or css.generate for all) to path and, with rasterize, each picture its rules refer to next to it (PNG of the object's visual bounds, 1 px per point) → {path, bytes, rules: n, images: [path…], warnings}; no path → {data: the CSS, name: <document>.css, rules, images: [{name, dataBase64}], warnings}",
+            "{path?, scope?: selection|all (default selection; ids? picks the objects), …options as css.selection, id?: object id (default: the selection)} write the CSS (css.selection, or css.generate for all) to path and, with rasterize, each picture its rules refer to next to it (PNG of the object's visual bounds, 1 px per point) → {path, bytes, rules: n, images: [path…], warnings}; no path → {data: the CSS, name: <document>.css, rules, images: [{name, dataBase64}], warnings}; warnings are css.selection's and the pictures' as document.export gives them (an image that couldn't be decoded, type in the fallback font or that doesn't fit its frame), each once",
             has_doc,
             export
         ),
@@ -100,12 +100,19 @@ fn export(s: &mut Session, p: &Value) -> Result<Value> {
     }
     let css = format!("{}\n", sheet.text());
     let doc = &s.doc()?.doc;
-    // The pictures of rasterized art.
+    // Encode the pictures of rasterized art, and add each of their warnings once.
     let mut images = Vec::new();
+    let mut warnings = sheet.warnings.clone();
     for r in &sheet.rules {
         let Some(name) = &r.image else { continue };
         let (d, _) = isolated(doc, &[r.id], name).ok_or_else(|| bad(C, format!("{} paints nothing to rasterize", r.selector)))?;
-        images.push((name, encode(&d, "png", &json!({}))?));
+        let (png, notes) = encode_with_warnings(&d, "png", &json!({}))?;
+        images.push((name, png));
+        for w in notes {
+            if !warnings.contains(&w) {
+                warnings.push(w);
+            }
+        }
     }
     let n = sheet.rules.len();
     match str_param(p, "path") {
@@ -118,12 +125,12 @@ fn export(s: &mut Session, p: &Value) -> Result<Value> {
                 write_file(&at, bytes)?;
                 written.push(at);
             }
-            Ok(json!({ "path": path, "bytes": css.len(), "rules": n, "images": written, "warnings": sheet.warnings }))
+            Ok(json!({ "path": path, "bytes": css.len(), "rules": n, "images": written, "warnings": warnings }))
         }
         None => {
             let images: Vec<Value> =
                 images.iter().map(|(name, bytes)| json!({ "name": name, "dataBase64": vectorcraft_format::base64_encode(bytes) })).collect();
-            Ok(json!({ "data": css, "name": default_name(doc, "css"), "rules": n, "images": images, "warnings": sheet.warnings }))
+            Ok(json!({ "data": css, "name": default_name(doc, "css"), "rules": n, "images": images, "warnings": warnings }))
         }
     }
 }