Animating Circle.set_fill results in a weird pattern.

Animating Circle.set_fill results in a weird pattern as follows:

https://user-images.githubusercontent.com/41336229/125058195-ac5fb800-e0c7-11eb-9a75-3f5ee05589de.mp4

Code:
```python
from manimlib import *

class Test(Scene):
def construct(self):
circle = Circle(color=BLUE).set_fill(BLUE, opacity=0.5)
self.play(ShowCreation(circle))
self.wait()
```

Also, how do I animate the fill smoothly?

Look at the Circle class in manimlib/mobject/geometry.py and the set_fill method. Investigate how fill animations are handled in the animation system, possibly in manimlib/animation/creation.py or transformation.py. Run the provided test code to reproduce the pattern, then examine the rendering pipeline to see how fill properties are interpolated during animation.

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