Use Boolean variable to store Boolean values.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78104 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Remove unused wxPG member variable.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78105 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775