Fix StrokeArrow: preserve tip geometry for large angles

[mathsvisualization]

Motivation

StrokeArrow did not preserve the tip geometry correctly for certain arc angles.
The arrow tips could disappear, distort, or scale incorrectly depending on the arc length.

Proposed changes

• Fixed condition in insert_tip_anchor to properly handle arc length and tip length.
• Replaced pointwise_become_partial with start_new_path to avoid distorted geometry.
• Updated create_tip_with_stroke_width to compute tip_width using original_stroke_width for consistent tip geometry.
• Removed redundant and buggy code paths.

Test

Code used:

from manimlib import *

class FixStrokeArrow(InteractiveScene):
    def construct(self):
        arrows = VGroup()
        for angle in [0, 45, 90, 135, 180]:
            arrow = StrokeArrow(LEFT, RIGHT, path_arc=-angle*DEGREES)
            arrows.add(arrow)
        arrows.arrange(DOWN)
        self.add(arrows)

Result

Before

After