Fix right margin of the carousel "next" button

[killthekitten]

The calculated offset of carousel's right-arrow differs with left-arrow on large viewport sizes. The problem was well described and illustrated here and here by @hnrch02. This PR is just a missing part of #13242

[mdo]

Sorry I missed that—why is this needed exactly? Are those numbers not equal on both sides without this?

[cvrebert]

Also, are we sure that it doesn't vary by browser?

[killthekitten]

@mdo visually they're not equal. That's very annoying if we use, for example, symmetrical background.

[killthekitten]

@cvrebert please, @hnrch02, could you recheck it in different browsers? Have no chance to do it by myself at the moment

[mdo]

I'm inclined to not merge this one small tweak given it's a 1px change and is (I think?) entirely dependent on using Glyphicons. If folks use something else, I'm guessing it's not an issue?

[hnrch02]

I'm not sure this has anything to do with Glyphicons, it's a problem with rounding in Chrome. If we take the carousel on the JavaScript page of the docs for example, the carousel container has a width of 816px, both .carousel-controls ought to have 15% of that 816px which would be 122,4px. Bizarrely, Chrome rounds down on the left control and up on the right one, hence the difference of one pixel and this hackfix. It doesn't seem to occur in Firefox, so I'd also vote against merging this.

[mdo]

Silly rounding. Probably fine for most other use cases though. For example, 15% of 940px (one of our grid tiers) is 141px. Given the carousel can be fluid, I see no reason to account for a small rounding error by the browser.