Bug 1085212: release video hardware before sharing and restore when done

Followup fix for bug 1088456: use playerShowing instead of playing so the patch works even if we are paused
I don't want to change playerShowing because we are leaving the player visible in this case, but just setting removing video.src.  playerShowing tells us whether we are showing the player or the thumbnails.  This is orthogonal to that.  We used to have release/restore on visiblity change, and it was just about video.src and never set playerShowing.

If I renamed needsRestore to videoReleased I think that would make the code clearer. (Though it would mean that the patch for 2.0 doesn't match anymore).