Fix InsetViewLayout.childGeometry

[Dark-Existed]

FIx #511

[Kyle-Ye]

Thanks for the fix.

A little suggestion before merging is:

If there is existing test case, turn it on (or remove the withKnownIssue logic). Otherwise, add a new UI test case for InsetViewModifierUITests.swift to verify it.

struct ContentView: View {
    var body: some View {
        Color.red
            .safeAreaPadding(.init(top: 10, leading: 20, bottom: 30, trailing: 40))
    }
}