Fix blocky selections on iOS Safari

In `domUtils.getBoundingClientRect`, for the iOS quirk `elementBCRIgnoresBodyScroll`, the returned rect did not have `width` and `height` properties.

This caused the `grownRect` inside `SvgSelectionView.getFillerRect()` to be undefined, because it depended on the 'grower' rects to have nonzero widths and heights.