Motivation / 修改原因

time_span doesn't work for animations of mobjects with submobjects, such as DashedLine.
像 DashedLine 这样含有多个子物件的图形的动画无法正确处理 time_span。

Proposed changes / 修改内容

• add method time_spanned_alpha in Animation to calculate the animation progress instead of changing rate_func when begin is called
  在 Animation 类中添加了计算动画进度的方法 time_spanned_alpha，专门用来计算存在 time_span 时的动画进度，而不在调用 begin 时改变 rate_func
• since Rotating also uses rate_func to decide the animation progress in interpolate_mobject, changes of rotation.py are made to correspond
  由于 Rotating 类的 interpolate_mobject 也是用 rate_func 计算动画进度，为了保持一致还修改了 rotation.py

Test / 测试

Code / 代码:

from manimlib import *


class TestScene(Scene):
    def construct(self) -> None:
        self.play(ShowCreation(Line((-1, 1, 0), (-1, -1, 0)), time_span=(0, 2)),
                  ShowCreation(DashedLine((1, 1, 0), (1, -1, 0)), time_span=(0.7, 2.7)))
        self.wait()

We hope the dashed line on the right side start to show later than the solid line on the left side.
右侧动画应比左侧晚开始。

Result / 结果:
After this change: (obviously the right one start later)
修改后：（右侧动画开始更晚）

Before this change: (two animations seem to start at the same time)
修改前：（两侧动画几乎同时开始）