True 2D Rendering

[smallmain]

Use Case

The engine lacks true 2D rendering, which is a very important feature. RenderRoot2D is actually in another rendering queue, and even with Depth Test enabled, it doesn't work, making many scenarios complicated or impossible to handle:

1. You need an additional separate camera to render 3D particle effects on top of 2D.

2. In scenarios where you need a character model - health bar (2D) - 3D particle effects, if there are occlusion relationships between different character models, it is currently impossible to achieve.

Although RenderTexture can solve the problem, it is too complex or has poor performance.

Similar issue: #17883

引擎缺少真正的 2D 渲染，这是非常重要的特性，RenderRoot2D 实际上在另一个渲染队列，即使开启了 Depth Test 也不行，导致许多场景的处理变得复杂或无法处理：

1.你需要多一个单独的相机才能在 2D 之上渲染 3D 粒子效果。

2.当需要 人物模型 - 血量（2d） - 人物本身的 3D 粒子效果 这样的排序场景时，如果不同人物模型之间还需有遮挡关系，在现在完全无法实现。

RenderTexture 虽然可以解决，但太复杂，或者性能不佳。

类似 issue：#17883

Problem Description

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Proposed Solution

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How it works

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Alternatives Considered

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Additional Information

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