Fix generic implementation of wxDC::GradientFillConcentric().

Fix generic implementation of wxDC::GradientFillConcentric().

The selected colour was not used as the code simply changed the value of
m_pen but didn't call SetPen() to actually use it. This resulted in always
using the default (black) colour when drawing concentric gradients.

Fix this by calling SetPen().

See #12337.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65943 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775