Reworked the mouse hiding routine. we were always hiding the mousecursor
  on kCGDirectMainDisplay, but this display is always the one with the
  menubar and not always the one containing our VLCVout.
  This new code takes the vout, and calculates on which displays (plural
  because of possible mirroring) it is being shown. It then shows or hides
  the cursor on all of those displays.
  This should fix #396.