Add a few missing helpers to SpriteAnimation

[luanpotter]

Description

I am porting Gravity Runner (aka Gravitational Waves) to the latest Flame.
I think it will be very good to have a fully functional, complete big game example before release.
Most of it was painless or even deleting stuff that we had incorporated from GR to Flame (like camera, viewport, etc).
However, I did come out with a list of things that were easier to do before. So I am putting out a few PRs.
This adds some useful features to SpriteAnimation.

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors and are passing (See Contributor Guide).
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• I updated/added relevant documentation (doc comments with ///) and updated/added examples in doc/examples.
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Breaking Change

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• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md).
• No, this is not a breaking change.