Layers: counting a row's art for its first thumbnail stops after 20,000 steps

A row drawn before was counted with no limit on its objects, so symbols
that each hold many instances of the next made the count grow with the
product of the instances, and the Layers panel could stall on such a file.

- Counting stops after 20,000 steps: each object, subpath, text run and
  symbol looked at is one step, and every 64 bytes of a symbol or placed
  file name looked up is one more. A row that needs more gets its
  thumbnail on the thumbnail thread, and the painted thumbnail draws
  nothing for it.
- A row drawn before counts at most 20,000 objects.

docs/target-app-parity.md

@@ -1274,7 +1274,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
   Delete, Cut, Group, Ungroup, Make and Release Compound Path, Make Clipping Mask, Send to Current Layer, Duplicate, Copy in the Move, Rotate, Scale, Reflect and Shear dialogs, Transform Each with Copy and each move of an Alt-drag take time in proportion to the size of the document (#1429).
   In the web build the status bar's render and UI times are measured with the browser's `performance.now()`, and the Layers panel stops rendering row thumbnails in a frame once they have taken 8 ms.
   On the desktop, the Layers panel renders a row's thumbnail on its thumbnail thread when the row's paths, and the paths of its opacity masks that are not disabled, hold more than 2,000 anchors in all. It already did this for the first thumbnail of a row of more than 200 objects. Until its first thumbnail is ready, the row shows boxes at the bounds of up to 41 of its paths, each in its path's solid fill or stroke color or in gray, when its own paths, without its opacity masks, hold at most 2,000 anchors in all, and an empty white box otherwise. With one path of 200,000 anchors, the first frame after the panel opens takes 2.4 ms instead of 393 ms (release build, median of three runs).
-  The same 2,000-anchor limit counts each glyph of type as 4 anchors, each point of a gradient mesh as 1 and every 100 pixels of an image as 1, and a symbol instance or a placed document counts the objects and anchors of the art it shows. A placed document whose file has not been read yet, or a symbol nested more than 8 deep (one that holds an instance of itself, for example), gets its first thumbnail on the thumbnail thread. The first frame after the panel opens takes 2.5 ms instead of 23.4 ms with 5,000 glyphs of point type, 2.1 ms instead of 346 ms with one instance of a symbol of a 200,000-anchor path, 1.7 ms instead of 11.7 ms with a 100 × 100 mesh, and 2.3 ms instead of 273 ms with a placed document of a 200,000-anchor path (release build, median of three runs).
+  The same 2,000-anchor limit counts each glyph of type as 4 anchors, each point of a gradient mesh as 1 and every 100 pixels of an image as 1, and a symbol instance or a placed document counts the objects and anchors of the art it shows. A placed document whose file has not been read yet, or a symbol nested more than 8 deep (one that holds an instance of itself, for example), gets its first thumbnail on the thumbnail thread, and so does a row whose art takes more than 20,000 steps to count (each object, subpath, text run and symbol looked at is one step). The first frame after the panel opens takes 2.4 ms instead of 19.8 ms with 5,000 glyphs of point type, 1.8 ms instead of 343 ms with one instance of a symbol of a 200,000-anchor path, 2.0 ms instead of 11.0 ms with a 100 × 100 mesh, and 2.1 ms instead of 274 ms with a placed document of a 200,000-anchor path (release build, median of three runs).
   In the web build, while a pan, zoom or rotation changes the view of a document whose last canvas render took more than 8 ms, the canvas draws the last frame moved with the view, and it renders the art again once the view has been still for 0.2 s.
   An Alt-drag with the Selection, Direct Selection, Group Selection, Rotate, Scale, Reflect or Shear tool copies the selection on its first move, and the moves after it transform those copies; the previews of the Move, Rotate, Scale, Reflect and Shear dialogs with Copy and Preview on copy the selection once in the same way (#1429).
   An Alt-drag of objects in perspective with the Perspective Selection tool, and an Alt-drag of a plane widget with that tool or the Perspective Grid tool, copy the objects on the first move, and the moves after it move those copies (#1429).