GridLayout does not work with computed child width/height

Testcase:

import QtQuick 2.2
import QtQuick.Layouts 1.3

GridLayout {
  columns: 3
  Repeater {
    model: 5
    Component.onCompleted: model = 6
    Rectangle {
      width: 50 + 0
      height: 50 + 0
      opacity: 0.5
      color: 'red'
      Text {
        text: index
      }
    }
  }
}

Note that changing width/height individually to just `50` produces different invalid results, but changing them both to just `50` produces expected result.

`model = 6` updates the items count. It doesn't necesarry have to increase the number of elements by 1, any change in the elements count triggers the bug here — you could verify that e.g. by setting `model = 2` there. It also doesn't have to be a number — using a ListModel and appending an element to it results in the same buggy behaviour.

`Text` item is irrelevant to the testcase and just demonstrates invalid positioning of the elements, e.g. in the case of `width: 50; height: 50 + 0`.

Note: while `50 + 0` is technically a static expression, it still triggers that bug, and this is thus the minimal testcase I was able to find (well, `(50)` also triggers the bug). The same issue happens, e.g. when there is some more complex expression like `someitem.width * 0.2`, etc — that should also work and should probably be included to the testcases.