Round up measurements of fixed-size stack child elements

[g-mark]

Without rounding up the measured size of Stack's fixed-size child elements, rounding errors and/or floating representations can result in layout issues with a child that should almost exactly fit in a stack, and these children are not being snapped to pixel edges.

This can be seen it in the added unit test. Consider this row:

Row {
    TestElement(width: 80.25).stackLayoutChild(priority: .fixed)
    TestElement(width: 80.25).stackLayoutChild(priority: .fixed)
}

When laid out on an @2x device, without the fix:

• child 1 has a width of 80.25, positioned at x = 0
• child 2 also has a width of 80.25, positioned at x = 80.25
• the row has a width of 161

After the fix, the row still has a width of 161, and:

• child 1 has a width of 80.5, positioned at x = 0
• child 2 also has a width of 80.5, positioned at x = 80.5
• cumulative width of children == width of the row