v1.5.14 — TIF 正射影像三级 LOD 金字塔

新增 / Added

中文：

TIF 正射影像转换为三级 LOD 金字塔 3D 平面：此前 TIF 转换只有两级（总览→高清），大尺寸影像在缩放过程中加载策略不够精细。现升级为与倾斜摄影同标准的三级金字塔结构：
- L0 总览：整幅缩到 ≤1024px 单张贴图，远视角只加载这一张，秒开不卡顿
- L1 中清：每块覆盖 2×2 个高清块，纹理 ≤1024px，中距离自动切换
- L2 高清：保持原分辨率（上限 16384px），网格块 ≤2048px，贴近后按视野加载所在高清块
层级切换复刻 Cesium 选瓦逻辑验证：远→中→近逐级过渡无跳变、无拉远消失（沿用 v1.5.12 的 GE 修复经验）
小图自动降级：影像较小（高清网格 ≤2×2）时自动退化为两级，单块时为单平面，行为与旧版一致
透明边缘处理保持 v1.5.13 修复：自带 alpha 通道直接沿用，无白边

English:

Three-level LOD pyramid for TIF orthophoto conversion: TIF planes previously had only two levels (overview → full-res). They now use the same three-level pyramid standard as oblique photogrammetry:
- L0 overview: whole image downscaled to ≤1024px, the only tile loaded at far distances — instant display
- L1 mid-res: each tile covers 2×2 high-res cells at ≤1024px, auto-selected at medium range
- L2 high-res: native resolution (up to 16384px) in ≤2048px grid cells; only cells in view load up close
Level switching verified against a replica of Cesium's tile-selection logic: no popping, no vanishing when zooming out (same GE fix as v1.5.12)
Small images degrade gracefully: ≤2×2 high-res cells fall back to two levels; single-cell images stay single-plane, same as before
Transparent-edge handling keeps the v1.5.13 fix: built-in alpha channel preserved, no white borders