cleanup previous fix for velocity scroll animations

[michaelknoch]

Type of change: cleanup fix

Motivation (current vs expected behavior)

This change is based on the previous feedback of @ephemer in https://github.com/flowkey/UIKit-cross-platform/pull/319. Instead of exposing the static transactionStack we can also just check if there is an animation prototype defined in UIView.currentAnimationPrototype. If this value is defined, we are currently in a stack trace of an animation closure.

The fix for the velocity scroll is to cancel all animations only when setting the contentOffset not from within an animation closure.

Please check if the PR fulfills these requirements

• [ ] Self-review: I am confident this is the simplest and clearest way to achieve the expected behaviour
• [ ] There are no dependencies on other PRs or I have linked dependencies through Zenhub
• [ ] The commit messages are clean and understandable
• [ ] Tests for the changes have been added (for bug fixes / features)

[codecov[bot]]

Codecov Report

Merging #320 (f2b1bce) into master (c8abdd2) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #320   +/-   ##
=======================================
  Coverage   51.15%   51.15%           
=======================================
  Files          87       87           
  Lines        2895     2895           
=======================================
  Hits         1481     1481           
  Misses       1414     1414           

Impacted Files	Coverage Δ
Sources/CATransaction.swift	78.57% <ø> (ø)
Sources/UIScrollView.swift	87.32% <100.00%> (ø)

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