Extra color rendering outside VMobjects caused by gradient

### Describe the bug

When rendering a complex `VMobject` with a gradient fill, unexpected colors may appear in certain areas.

**Code example**:

```python
class Test(Scene):
def construct(self):
tex = Tex(R"2").set_height(7).set_color([BLUE, GREEN])
self.add(tex)
```

**Observed behavior**:
Instead of a smooth gradient, some regions display incorrect or “extra” colors.

**Screenshot**:

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### Root cause analysis

This appears to be related to how `VMobject` fills are implemented.
Currently, the fill is constructed by blending a sequence of triangles, each defined by one **fixed vertex** and two consecutive vertices along the shape’s boundary (i.e., the 1st, *i*-th, and (*i*+1)-th points).

When a gradient is applied, overlapping triangles end up with slightly different colors. Since overlapping regions use blending with negative opacity, the colors don’t fully cancel out, leaving visible artifacts.

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### Possible fix

Instead of the current “fan” triangulation approach, use a proper polygon triangulation algorithm to generate non-overlapping triangles for rendering. This should eliminate color inconsistencies caused by overlapping regions.

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### Additional Content
System info:
- ManimGL 1.7.2
- Python 3.12.9
- Windows 11
- GPU: Intel Arc Pro Graphics (Driver 32.0.101.6637)

查看 VMobject 的 fill 實作，可能在 manim/mobject/types/vectorized_mobject.py 或相關圖形模組中。瞭解目前用於 gradients 的「fan」triangulation。研究可用來替換它的 polygon triangulation 演算法（例如 ear clipping）。使用提供的程式碼範例進行測試，查看 artifact 並驗證修正是否消除 extra colors。

由索引模型根據 Issue 內容生成。