wxPanel didn't inherit the background colour set for its parent any more in
wxMSW because its HasTransparentBackground() didn't return true unless the
panel was a child of wxNotebook. This was wrong because not only themed
notebook background should be inherited but also any solid background
explicitly set for a panel parent.
Fix this by returning true from MSWHasInheritableBackground(), which is used
by wxPanel::HasTransparentBackground() under MSW, if the window has an
explicitly set and inheritable background colour.
Closes#13487.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69348 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
4506b42ce9