How to fill with gradient in shader mode?

How to fill circle for example with gradient in shader mode? I tried set_color_by_gradient, it results in having different colors for different submobjects.

For example:

```
class Gradient(Scene):
def construct(self):
circle_1 = Circle().move_to(2*RIGHT)
circle_2 = Circle().move_to(2*LEFT)
circles = VGroup(circle_1,circle_2)
self.add(circles.set_color_by_gradient(PINK, YELLOW))
self.wait()
```

The results is:

https://user-images.githubusercontent.com/53943014/102837108-1a97a280-4436-11eb-83cf-72dee1fa1d3b.mp4

What I want is something like this:


All suggestions are highly appreciated. Thank you.

此問題是將 gradient fill 應用於 shader mode 中的單一形狀，而不是跨越多個 submobjects。查看 manim 的 shader mode 實作，尤其關注顏色與 gradient 在渲染管線中的處理方式。檢查 set_color_by_gradient 方法和相關 shader 程式碼，以理解目前行為。目標是修改 shader，使其在單一物件（如 circle）內套用 gradient。

由索引模型根據 Issue 內容生成。