CLIP on raster images and viewports, IMAGECLIP, VPCLIP, clip depth rules

- CLIP continues as IMAGECLIP on a raster image and as VPCLIP on a layout
  viewport, with the reference prompts and stored data.
- Image clip boundaries sit on pixel centres, OFF only clears the display
  flag, and an inverted boundary cuts the image.
- VPCLIP polygon (Arc/Length/Close/Undo), clipping object and Delete; the
  viewport shrinks to the boundary's exact extents, view kept.
- XCLIP Clipdepth: Remove clears only the plane asked, a front plane behind
  the back one is rejected with the reference message.
- Attached and imported images carry the reference display flags.

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

src/scene/pick/xclip.rs

@@ -169,13 +169,22 @@ pub fn set_insert_clip_enabled(doc: &mut CadDocument, insert: Handle, on: bool)
 }
 
 /// Front and back clipping planes (None = off) of an insert's clip.
-pub fn set_insert_clip_depth(doc: &mut CadDocument, insert: Handle, front: Option<f64>, back: Option<f64>) -> bool {
+pub fn set_insert_clip_depth(
+    doc: &mut CadDocument,
+    insert: Handle,
+    front: Option<Option<f64>>,
+    back: Option<Option<f64>>,
+) -> bool {
     let Some(spatial) = filter_handle(doc, insert) else {
         return false;
     };
     if let Some(ObjectType::SpatialFilter(f)) = doc.objects.get_mut(&spatial) {
-        f.front_clip = front;
-        f.back_clip = back;
+        if let Some(front) = front {
+            f.front_clip = front;
+        }
+        if let Some(back) = back {
+            f.back_clip = back;
+        }
     }
     true
 }