Use better logic to generate rounded rectangle geometry in RoundedClipConverter

[foglio1024]

https://github.com/Foglio1024/Nostrum/blob/deda86ec2c43710624468f541e3ee4ef76aac5fe/Nostrum.WPF/Converters/RoundedClipConverter.cs#L26-L28

See https://github.com/ratishphilip/wpfspark/blob/master/WPFSpark/Common/ClipBorder.cs