Fix rounding error when calculating element width

[anteq]

When width of rootElm is not an integer, the decimal part is being lost in the width calculation. Rounding of width may cause to display the carousel improperly (showing 1px of next image).
According to the Mozilla developer docs https://developer.mozilla.org/en-US/docs/Web/API/Element/clientWidth , proper solution to get exact width is usage of getBoundingClientRect().
Can be used safely in modern browsers: https://caniuse.com/#feat=getboundingclientrect

[ZouYouShun]

Thanks a lot, @anteq

[anteq]

Cool, no problem! Can we expect that to be included in the release soon?

[anteq]

Hey @ZouYouShun, can you estimate when you'll be able to release it onto npm? Looking forward to it since its an issue that affects our usage.

Thank you, really appreciate the work put into this library!