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 <noreply@anthropic.com>

src/command.rs

@@ -1938,6 +1938,9 @@ pub enum CmdResult {
     MviewCreateClipped {
         boundary: Option<EntityType>,
         boundary_handle: Handle,
+        /// VPCLIP: the viewport to clip instead of creating one (NULL for
+        /// MVIEW). With neither boundary nor handle its clip is deleted.
+        target: Handle,
     },
     /// Create a wipeout from an existing closed polyline in the active space.
     /// `erase_source` controls whether the source boundary is consumed.