Fix implicit floating point precision loss

Also wondering wether `newRect.origin.y += (heightDelta / 2);` on [line 413](https://github.com/rastersize/Butter/blob/d66ab3a237948649db63e993bf76da9c4efdfac1/Butter/BTRSecureTextField.m#L413) in `BTRSecureTextField` should also be ceiled like it is in `BTRTextField` (line 542)? If so just let me know and I’ll fix that in this pull request as well.

:surfer: