OverlayClient: hide QGraphicsPixmapItems when FPS and time are disabled instead of setting an empty QPixmap.
OverlayClient: hide QGraphicsPixmapItems when FPS and time are disabled instead of setting an empty QPixmap.

Setting an empty QPixmap, at least in Qt 5, causes a great deal of
redraws. The QGraphicsScene triggers a changed() call, and
OverlayClient sends a blit request to the overlay library.

Before, any update to the invisible FPS counter or time view
would cause a full screen blit.

Now, when the overlay is truly empty (no FPS, time or users), no
messages are sent over the wire to the overlay library.

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