PopupWidget: use translateZ(0) on drop-shadows in wikimediaui
PopupWidget: use translateZ(0) on drop-shadows in wikimediaui

This avoids a Safari bug which can leave behind visual glitches when hiding
popups.

Bug: T226872
Change-Id: I19c6066d4b7a80c5d2484a21b19bb5fac200531a

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui