even controls with transparent background should draw it if they have an explicitly...

even controls with transparent background should draw it if they have an explicitly set background colour, so handle this at wxWindowUniv level instead of adding hacks to individual controls, such as wxStaticText, to deal with this (replaces patch 1700004)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@45446 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775