fix paint for standalone buttons with dynamic size

m_iconSize for standalone buttons can be changed dynamically based on user or container preferences. Such an example could be the window buttons applet that is resizing its buttons based on panel thickness or when the applet is on the desktop and the user resizes it. Previous implementation was failing because even though m_iconSize was set as NULL for StandAlone buttons, on the first painting m_iconSize was set based on Button::geometry(). This of course was failing because the Button::geometry() could change afterwards and painting was not considering it to calculate m_iconSize again.