Fix Line failing with buff and path_arc

[irvanalhaq9]

Overview: What does this pull request change?

Fixes issue: #4132

Motivation and Explanation: Why and how do your changes improve the library?

Issue Explanation

When buff is given, Line internally calls pointwise_become_partial, passing self as the argument for vmobject:

self.pointwise_become_partial(vmobject=self, a=buff_proportion, b=1 - buff_proportion)

In Manim v0.19.0, the pointwise_become_partial code was modified in commit 374eeeba (from PR #3760). This change introduced the issue.

Initially (in my first commit), I addressed this issue by modifying pointwise_become_partial so that it creates a copy of vmobject.points if vmobject is self, avoiding in-place modification of vmobject.points when self.points is set to np.empty(...). Making a copy is essential because self.points and vmobject.points are used together after self.points is set to np.empty(...). See the related source code.

However, I realized that a simpler solution exists which avoids modifying pointwise_become_partial. Instead, the issue can be resolved by adjusting how Line uses it: When calling it, instead of passing vmobject=self, we should pass vmobject=self.copy(). I’ve implemented this in my most recent commit.

Reviewer Checklist

• The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• If applicable: newly added functions and classes are tested