Carousel uses bad value for perspective property

Created by: cvrebert

https://github.com/twbs/bootstrap/blob/2e6511bb51d23b1f54d2a2ed32282e4a50ba959a/less/carousel.less#L32 ends up compiling (in part) to perspective: 1000; However, per the spec and https://developer.mozilla.org/en-US/docs/Web/CSS/perspective , perspective's value must be either none or a length. Non-0 lengths must have a unit, so the unitless 1000 is invalid. CC: @mdo Originally reported as https://github.com/twbs/bootstrap-sass/issues/882