Fixes for EXT-8563 "VWR-20156: Prim Media on HUDs has no Audio or Interactivity"

Backed out changeset 1b65d0d42c67 (the fix for EXT-5205), and replaced it with a check in LLPanelPrimMediaControls::nextZoomLevel().

Made LLViewerMediaImpl::calculateInterest() not attempt to calculate distances to HUD attachments, since their global positions are invalid.