Artboards: each artboard can have a background color on the canvas

Squashed from #2403 onto main.

crates/engine/src/cmd/fileio/encode.rs

@@ -340,7 +340,7 @@ pub(super) fn single_artboard(doc: &Document, rect: Rect, name: &str) -> Documen
 
 /// `d` with one artboard, `rect`.
 pub(crate) fn with_single_artboard(mut d: Document, rect: Rect, name: &str) -> Document {
-    d.artboards = vec![Artboard { id: 1, name: name.into(), rect, show_center_mark: false, show_cross_hairs: false }];
+    d.artboards = vec![Artboard { id: 1, name: name.into(), rect, show_center_mark: false, show_cross_hairs: false, color: None }];
     d
 }