WebGL2 异步读取纹理数据速度很慢

[xiaoiver]

目前异步 readTexture 方法使用了 gl.fenceSync，按说是比较推荐的非阻塞做法（Babylon.js 也使用了该做法），但在 L7 中实测相比同步方法要慢很多。

https://developer.mozilla.org/en-US/docs/Web/API/WebGL_API/WebGL_best_practices#use_non-blocking_async_data_readback

https://github.com/BabylonJS/Babylon.js/blob/master/packages/dev/core/src/Engines/engine.ts#L1016C12-L1016C28
https://github.com/BabylonJS/Babylon.js/blob/master/packages/dev/core/src/Engines/Extensions/engine.readTexture.ts#L179

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Title: WebGL2 asynchronous reading of texture data is slow

Currently, the asynchronous readTexture method uses gl.fenceSync, which is said to be a more recommended non-blocking method (Babylon.js also uses this method), but in L7 it is much slower than the synchronous method.

https://developer.mozilla.org/en-US/docs/Web/API/WebGL_API/WebGL_best_practices#use_non-blocking_async_data_readback