View backgrounds: reference blend, image adjust, layouts, cleanup, style name

- Gradients: the middle colour sits (horizon - 0.5) viewport widths from the
  centre and blends over height viewport heights each side; two colours
  blend over a viewport height each side.
- Image backgrounds: offset in device pixels (right, down), negative scale
  mirrors, fitted with aspect ratio covers the viewport; the dialog keeps an
  image's offset and scale.
- Layouts: a model-space view restored on a sheet asks for the viewport,
  enters it and gives it the view's camera, background and visual style;
  inside a viewport the view goes there; a paper-space view's background is
  kept on the sheet viewport but not drawn; floating viewports draw their
  background.
- Background objects no view or viewport uses any more are erased; a view
  without a background restored where the viewport already is leaves the
  viewport's background.
- The viewport's named visual style (Realistic, ...) shows by name and the
  render-mode picker records its style on the viewport record.

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

src/modules/view/visual_style.rs

@@ -106,6 +106,38 @@ pub fn label_for(mode: Mode) -> &'static str {
         .unwrap_or("Wireframe 2D")
 }
 
+/// The render mode a named visual style draws as, and whether the name is a
+/// style of its own (Realistic, Conceptual, …) rather than one of the modes.
+pub fn mode_for_style_name(name: &str) -> Option<(Mode, bool)> {
+    Some(match name {
+        "2dWireframe" => (Mode::Wireframe2D, false),
+        "Wireframe" => (Mode::Wireframe3D, false),
+        "Hidden" => (Mode::HiddenLine, false),
+        "Flat" => (Mode::FlatShaded, false),
+        "FlatWithEdges" => (Mode::FlatShadedWithEdges, false),
+        "Gouraud" => (Mode::GouraudShaded, false),
+        "GouraudWithEdges" => (Mode::GouraudShadedWithEdges, false),
+        "Shaded with edges" => (Mode::GouraudShadedWithEdges, true),
+        "Realistic" | "Shaded" | "Conceptual" | "Shades of Gray" | "Sketchy" | "X-Ray" => {
+            (Mode::GouraudShaded, true)
+        }
+        _ => return None,
+    })
+}
+
+/// The name of the visual style a render mode is saved as.
+pub fn style_name_for(mode: Mode) -> &'static str {
+    match mode {
+        Mode::Wireframe2D => "2dWireframe",
+        Mode::Wireframe3D => "Wireframe",
+        Mode::HiddenLine => "Hidden",
+        Mode::FlatShaded => "Flat",
+        Mode::FlatShadedWithEdges => "FlatWithEdges",
+        Mode::GouraudShaded => "Gouraud",
+        Mode::GouraudShadedWithEdges => "GouraudWithEdges",
+    }
+}
+
 /// The style a style keyword names. Only the seven exist; nothing maps onto a
 /// nearest neighbour, because there is no longer anything else to map from.
 pub fn mode_for_keyword(keyword: &str) -> Option<Mode> {