Maintain the same visible items after rotation

[bryankeller]

Details

This makes it so that MagazineLayout will maintain a similar scroll position after rotation. It does this by introducing some additional bookkeeping so that we know which item was closest to the top before the rotation. We then ensure that our target content offset positions this item at the top after rotation.

As usual, UICollectionViewLayout is an undocumented, confusing mystery. 4 calls to prepare(forAnimatedBoundsChange oldBounds:) for one rotation? Makes sense!

Related Issue

N/A

Motivation and Context

Slight behavior improvement - nothing major.

How Has This Been Tested

Tested on iOS simulator; iPhone and iPad.

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

[brynbodayle]

Good stuff!

[bryankeller]

Merging now that CI is fixed (and passing)