[BUG] Item's with different size does not work as expected for swipeDirection vertical

[suhaiyl]

Describe the bug
Item's with different size does not work as expected

To Reproduce

• try to set different preferred size for items
• Sample code
• preferredItemSize(CGSize(width: 300, height: self.page1.index % 2 == 0 ? 350 : 450))

Expected behavior
A clear and concise description of what you expected to happen.
The items should be loaded with size as expected

Screenshots / Videos
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Environment:

• OSX: [e.g. iOS]
• Device [e.g. iPhone6]
• SwiftUIPager version

Additional context
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