fix: 缓存的 BlitDesc._blit 更新

[Magnetismmy]

Re: #

Changelog

缓存的 [BlitDesc._blit]与对象池复用机制不兼容，导致在 shadingScale 改变 blit 分配顺序后读取到失效/被复写的 blit 对象。

复现思路 ：多 pass 混合
场景里同时存在：
fullscreen 后处理 blit
UI 渲染
shadingScale 引入的缩放 pass
多帧运行后，用多个不同后处理配置的相机，从A开启切换到B开启，某帧中对象池 slot 被复用
旧 [BlitDesc._blit]指向了被重置后的 UI blit，触发空材质访问

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Continuous Integration

This pull request:

• needs automatic test cases check.

  Manual trigger with @cocos-robot run test cases afterward.

• does not change any runtime related code or build configuration

  If any reviewer thinks the CI checks are needed, please uncheck this option, then close and reopen the issue.

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Compatibility Check

This pull request:

• changes public API, and have ensured backward compatibility with deprecated features.
• affects platform compatibility, e.g. system version, browser version, platform sdk version, platform toolchain, language version, hardware compatibility etc.
• affects file structure of the build package or build configuration which requires user project upgrade.
• introduces breaking changes, please list all changes, affected features and the scope of violation.

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[github-actions]

Code Size Check Report

Wechat (WASM)	Before	After	Diff
2D Empty (legacy pipeline)	1009738 bytes	1009738 bytes	✅ 0 bytes
2D All (legacy pipeline)	2675347 bytes	2675347 bytes	✅ 0 bytes
2D All (new pipeline)	2767083 bytes	2767098 bytes	⚠️ +15 bytes
(2D + 3D) All	10024081 bytes	10024096 bytes	⚠️ +15 bytes

Web (WASM + ASMJS)	Before	After	Diff
(2D + 3D) All	16843143 bytes	16843158 bytes	⚠️ +15 bytes

Interface Check Report

This pull request does not change any public interfaces !