Extend wxBannerWindow background bitmap if necessary.

Extend wxBannerWindow background bitmap if necessary.

Ensure that the entire banner window uses the same background colour as the
bitmap, even when the bitmap is too small for the window size. This allows to
avoid using huge bitmaps if extending the bitmap with solid colour looks good
enough.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69859 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775