Don't let the background become smaller than the svg

Otherwise the svg rendering results in broken looks.
BUG:424448

src/declarativeimports/plasmacomponents3/ToolTip.qml

@@ -40,6 +40,8 @@ T.ToolTip {
     }
 
     background: PlasmaCore.FrameSvgItem {
+        height: Math.max(implicitHeight, control.height)
+        width: Math.max(implicitWidth, control.width)
         imagePath: "widgets/background"
     }
 }