fix(position): correct positioning for SVG elements

Bootstrap tooltips calculate the `width` and `height` of elements
using first getBoundingClientRect and fall back to `offsetWidth` and
`offsetHeight` if it is not available:

https://github.com/twbs/bootstrap/blob/master/js/tooltip.js#L297

This fixes a problem with tooltips/popovers on SVG elements in
Firefox: https://github.com/twbs/bootstrap/issues/5956

This change mimics that behavior.

Closes #1225