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

include/wx/bannerwindow.h

@@ -112,6 +112,10 @@ private:
     // configurable in the future.
     wxFont GetTitleFont() const;
 
+    // Return the colour to use for extending the bitmap. Non-const as it
+    // updates m_colBitmapBg if needed.
+    wxColour GetBitmapBg();
+
 
     // The window side along which the banner is laid out.
     wxDirection m_direction;
@@ -119,6 +123,10 @@ private:
     // If valid, this bitmap is drawn as is.
     wxBitmap m_bitmap;
 
+    // If bitmap is valid, this is the colour we use to extend it if the bitmap
+    // is smaller than this window. It is computed on demand by GetBitmapBg().
+    wxColour m_colBitmapBg;
+
     // The title and main message to draw, used if m_bitmap is invalid.
     wxString m_title,
              m_message;