Properly hide checkbox and radio inputs in button groups

`pointer-events: none;` for modern browsers (including IE11+), `clip: rect(1px, 1px, 1px, 1px);` for everything else.

Maybe @mdo can give some input on `clip`, I found various resources online promoting `1px, 1px, 1px, 1px` instead of `0, 0, 0, 0`.

Fixes #14137.
Per https://github.com/twbs/bootstrap/issues/14137#issuecomment-54754214.