Vanishing Point: on-canvas plane editor with Create/Edit Plane, grid and paste (#3122) (#3133)

Filter > Vanishing Point now opens a dialog mode on the canvas instead of
the generic parameter dialog:

- Create Plane (C): four clicks, in any order, make a plane (corners are
  normalised clockwise from the top-left, the order paste expects).
- Edit Plane (V): drag corner nodes, or edge nodes to slide an edge along
  the plane's own perspective; click a plane to select it, Delete removes it.
- The plane grid is drawn at the Grid Size, in the accent colour when the
  plane is valid, warning yellow when its perspective is doubtful and red
  (outline only) when the quad isn't convex (VpPlane::check).
- With an image on the clipboard the selected plane previews it pasted in
  perspective; drag to move it, Width sets its size, New Layer renders it
  on a new layer. OK / Enter runs filter.vanishingPoint with the planes and
  the paste as one history step; the control channel drives the same
  session through {"ui": {...}}.

The engine now rejects a drawn plane whose corners don't form a convex
quad (bow-tie, dented) instead of warping along it.

Co-authored-by: Claude Opus 5.5 <[email protected]>
Co-authored-by: Brandon Thomas <[email protected]>

scorecard/tools.toml

@@ -206,7 +206,7 @@ id = "TOOL-214-14"
 target = "Vanishing Point window: draw and edit planes, paste, clone in perspective"
 status = "partial"
 issue = 214
-note = "filter.vanishingPoint (vp_cmds.rs:15) behind a parameter dialog"
+note = "filter.vanishingPoint (vp_cmds.rs:15); on-canvas plane editor (vp_ui.rs: Create/Edit Plane, edge nodes, valid/doubtful/invalid grid colours, Grid Size, clipboard pasted onto the plane on OK); clone stamp, marquee and tear-off planes in the window still missing"
 
 [[item]]
 id = "TOOL-214-15"