set 50% as maximum for background radius

--according to https://www.w3schools.com/howto/howto_css_rounded_images.asp
50% is considered the default maximum value for background
roundness.

containment/package/contents/ui/background/MultiLayered.qml

@@ -198,8 +198,8 @@ BackgroundProperties{
         }
 
         return plasmoid.formFactor === PlasmaCore.Types.Horizontal ?
-                    (plasmoid.configuration.backgroundRadius/100) * (solidBackground.height/2) :
-                    (plasmoid.configuration.backgroundRadius/100) * (solidBackground.width/2)
+                    (plasmoid.configuration.backgroundRadius/100) * solidBackground.height :
+                    (plasmoid.configuration.backgroundRadius/100) * solidBackground.width
     }
     readonly property int customShadow: {
         if (customDefShadowIsEnabled && themeExtendedBackground) {

shell/package/contents/configuration/pages/AppearanceConfig.qml

@@ -923,7 +923,7 @@ PlasmaComponents.Page {
 
                         value: plasmoid.configuration.backgroundRadius
                         from: -1
-                        to: 100
+                        to: 50
                         stepSize: 1
                         wheelEnabled: false