Reduce `getBoundingClientRect()` calls

[lpimem]

Element.getBoundingClientRect() triggers a repaint of the whole dom tree and should be avoided.

Related code:

rangeblock/src/core/node_measure_impl.ts

Line 132 in 25e7d34

let rect = e.getBoundingClientRect();

rangeblock/src/core/node_measure.ts

Line 55 in 25e7d34

measureSpanToDim(span, doc, (doc: Document, el: HTMLElement)=>{

[lpimem]

consider using Element.offsetParent to do the calculation.