Disable alpha transformation if opacity is 100%

IE 8 does not display overflowing content on elements that have both a z-index
and an alpha channel transformation (opacity).

Since disabling the alpha channel transformation is visually equivalent
to setting it to 100%, we may as well just disable it in these cases.
Merge remote-tracking branch 'lencioni/opacity'

Conflicts:
	core/stylesheets/compass/css3/_opacity.scss