-VIEW sEttings Background dialog and view backgrounds

Color, Gradient and Image at the background prompt open a Background
dialog (None / Solid / Gradient / Image, preview, solid colour, two- or
three-colour gradient with rotation, image file with position and
scale). Apply links the view to a new background object in
ACAD_BACKGROUND; restoring a view gives the model viewport a copy of
it (or none), and saving a view copies the viewport's background.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/scene/mod.rs

@@ -28,6 +28,7 @@ pub enum BlockScalePolicy {
 // blocks and/or free functions). Pure text-move from the original mod.rs.
 mod boundary;
 mod camera_ops;
+pub use camera_ops::{add_background, copy_background};
 pub(crate) mod centerline;
 pub(crate) mod centermark;
 pub(crate) mod dimension_assoc;