Fix unbounded tree node growth in XYZ globe image tiles

[WilliamLiu-1997]

Tree Node Memory Leak in XYZ Globe Image Tiles

Relate to #1554

Problem

When using ImageFormatPlugin (XYZ / TMS / WMS image tile sources) on a globe, the tile node tree grows unboundedly as the camera pans. Unlike standard 3D Tiles where the tree structure is finite and defined by tileset.json, image tile trees are infinite and dynamically generated — ImageFormatPlugin.expandChildren creates child nodes on the fly as the quadtree subdivides to meet the screen-space error target.

The root cause is that ensureChildrenArePreprocessed(tile) was called inside resetFrameState (optimized path) and markUsedTiles / recursivelyMarkUsed (non-optimized path). This means every tile visited during traversal unconditionally triggered child creation, even when the traversal ultimately decided not to descend. As the camera panned across the globe, children were created for every visited tile but never reclaimed — the tree grew monotonically.

The leak is in node objects themselves, not in content/textures/geometry (those are managed by the LRU cache). Over time, the accumulated tile node objects cause significant memory growth that does not recover even after returning to a previous view.

Fix

The fix has two parts:

1. Lazy expansion — Move ensureChildrenArePreprocessed from the "visit" phase (resetFrameState / markUsedTiles) to the "descend" phase (canTraverse). Children are only created when the traversal actually decides to refine deeper (error target not yet met). This is a change in the core traversal files and applies to all tile types.

2. Subtree pruning — After each traversal, ImageFormatPlugin walks the tree and removes subtrees where no descendant is retained (not in the LRU cache, not used, not visible, not active). expandChildren is made idempotent (deduplicates against existing children) so that pruned subtrees can be safely re-created on demand when the camera returns. A _imageChildrenComplete flag provides a fast path to skip the dedup check on steady-state frames.

Why this only affects ImageFormatPlugin

	Standard 3D Tiles	Image Tiles (ImageFormatPlugin)
Tree size	Finite, defined by tileset.json	Infinite, dynamically generated
Children source	JSON parsing (one-time)	expandChildren in preprocessNode (on demand)
Can recreate children	No (would need to re-fetch JSON)	Yes (expandChildren is idempotent)
Node leak	No	Yes (fixed by this change)

Other plugins that dynamically create children (ImplicitTilingPlugin, QuantizedMeshPlugin, ImageOverlayPlugin) do so in parseTile (after content loads), not in preprocessNode, so they are not affected by the eager expansion in resetFrameState. Their child lifecycle is already managed through LRU eviction callbacks.