Restore the media focus ring. This fixes DEV-42166.

Media focus is still tracked separately from LLSelectMgr (which was a change made during a refactor a while ago), but LLViewerMediaFocus now keeps LLSelectMgr updated with the current media focus when it changes.

Added a special case for media focus to LLSelectMgr::deselectAllIfTooFar(), since it was making the focus ring not show up when media focus was on distant media.