qml: add `effectiveRadius` property to `RoundImage.qml`

Rounding is done within the shader now. Since shaders
are only supported with RHI now, there is no rounding
when RHI is not used. In that case, we can represent
the effective radius with a read-only `effectiveRadius`
property which would be 0 with non-RHI (software or
openvg mode) cases.

modules/gui/qt/widgets/qml/RoundImage.qml

@@ -38,6 +38,7 @@ Item {
     // so it is not provided as an alias here
 
     property real radius
+    readonly property real effectiveRadius: (shaderEffect.readyForVisibility ?? shaderEffect.visible) ? radius : 0.0
 
     // NOTE: Note the distinction between ShaderEffect and
     //       ShaderEffectSource. ShaderEffect is no different