Make DesktopOverlay use public getter instead of explicit settings call

Using the getter is clearer and leads to less code duplication.

src/openvr/overlay_utils.cpp

@@ -31,8 +31,7 @@ void setDesktopOverlayWidth( double width )
 
 void DesktopOverlay::update()
 {
-    setDesktopOverlayWidth(
+    setDesktopOverlayWidth( getCurrentWidth() );
-        settings::getSetting( settings::DoubleSetting::UTILITY_desktopWidth ) );
 }
 
 bool DesktopOverlay::isAvailable() const