Rendering transparent png graphics edges will blend the background color

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## Description of bug / unexpected behavior
I drew a circle in the scene, added the -t parameter when rendering, and then added a background to the picture. I found that the edge of the graphic was mixed with the background color.

## Expected behavior
Graphic edges should not blend with the background color

## How to reproduce the issue

Code for reproducing the problem

```py
from manim import *

config.pixel_width = 400
config.pixel_height = 400
config.frame_height = 400.0
config.frame_width = 400.0

# 传入一个像素宽度，返回Manim单位的线条粗细
def calculate_stroke_width(pixelWidth):
# 先计算Manim单位的像素宽度
unitPixel = config.pixel_width / config.frame_width
# 然后计算线条的Manim单位粗细，线条粗细单位是Manim单位的0.01
strokeWidth = pixelWidth / unitPixel * 100
return strokeWidth

# 创建一个主场景继承至场景类
class MainScene(Scene):
# 定义构造方法
def construct(self):
# 绘制圆形
circle = Circle(radius=100, stroke_width=calculate_stroke_width(4), color=WHITE)
self.add(circle)
```

## Additional media files

Images/GIFs

## System specifications

System Details

- OS (with version, e.g., Windows 10 v2004 or macOS 10.15 (Catalina)):
- RAM:
- Python version (`python/py/python3 --version`):
- Installed modules (provide output from `pip list`):
```
OS: Windows10 Home 22H2
RAM: 48GB
Python version: 3.12.0
```

## Additional comments

I noticed that some users have reported this issue before, and you have fixed it. #2273

Start by running the provided Python reproduction with the Scene, Circle, and -t transparent-rendering settings, then compare the PNG edge pixels before and after adding a background. Trace the rendering and alpha-compositing path involved in transparent output; done means graphic edges remain independent of the later background color.

Written by the indexing model from the issue text.