Don't use m_UseBkColour flag which remained always set once
SetBackgroundColour() had been called, but just rely on m_BkColour itself
being valid to determine whether we should use it. This allows to reset the
background colour after setting it.
Closes#15287.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74306 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775