Fix for #2488. Use element offsetHeight rather than getBoundingRect

[arekouzounian]

This was a simple fix for #2488, as using the offsetHeight/Width resolved the issue.

I tried to create a quick test for the patch by having a button that would apply the transform to the body, but applying the transform after the page had loaded didn't cause any issues with character size, even without the patch applied.

I decided just to leave out the button, because it didn't seem to help as a test.

This is my first open-source contribution ever, so any and all feedback is much appreciated. Thank you!

[Tyriar]

Thanks @arekouzounian! Sorry about taking so long for your first ever contributor

[jerch]

Eww, looking at this PR again - plz note that offsetWidth & offsetHeight are defined as integer, where getBoundingClientRect gives fractions. Ofc ideally we want the cells to be spaced in integers, but I am not sure if the dimension calculations in the renderers still will yield the same numbers here.
To get more accurate fractional numbers we'd have to measure multiple chars at once (already applied for width in the DOM PR).

Edit:
Still gonna change it in the width cache to match this (as I did not find any glitches from it), but we prolly should address this with a follow-up.