Make reading element dimensions work regardless of transforms

[davidjerleke]

Feature request is related to

• embla-carousel (core package)

Is your feature request related to an issue?

• [Help]: Animations add incorrect slides offset, but only sometimes #346

^{Originally posted by javiergonzalezGenially}
When using the carousel inside a container with a scale transform applied the carousel doesn't work properly because it uses to measure nodes getBoundingClientRect, which returns a rect which is already scaled.

Describe the solution you'd like

^{Originally posted by javiergonzalezGenially}
The proposed fix (tested working in the playground) uses instead offsetLeft/offsetWidth etc, which are unscaled.