Don't always override the default background colours.

Don't always override the default background colours.

The wxColour argument taken by wxControl::DoMSWControlColor() allows to
override the default colour, it shouldn't be used in MSWControlColor() as this
resulted in always using custom colours for the controls, even when the user
hadn't changed them. Fix this by not passing any valid colour to it in this
case and allowing it to deduce the correct colour to use on its own.

See #1691.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61859 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775