Changed TShowCaseCtrl not redraw itself when invalidated. This prevents it from overwriting any showcased control.
This fixes bug #3559156 (http://bit.ly/TKXYQ7): "Previews" disappearing in Preferences dialogue box." in trunk.
Authored by: delphidabbler 2012-08-18
Parent: [r2225]
Child: [r2227]
