Fix unintended propagation of kwargs in LaggedStartMap

[irvanalhaq9]

Overview: What does this pull request change?

Fix #4508

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

Fix unintended kwargs propagation in LaggedStartMap
Ensure kwargs intended specifically for animation_class are not forwarded to the superclasses of LaggedStartMap, and make lag_ratio explicit."

[behackl]

This seems reasonable to me, thanks for the fix!

Do you think it makes sense to add some kind of explicit test for this?

[behackl]

Proposed one test in a reviewer commit. After writing/reading this again, it does feel a bit odd to read

anim = LaggedStartMap(..., rate_func=linear)
assert anim.rate_func is smooth

Is this fine? Should we improve it in a different way?

[irvanalhaq9]

@behackl Thanks for reviewing. I have updated the test.