Set m_inheritBgCol flag in wxPanel with a background bitmap.

This ensures that the panel background is used by its transparent children
under wxMSW even when the panel itself is inside a wxNotebook with themed
background: making InheritsBackgroundColour() return true prevents the
notebook from overriding our background.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67304 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/panel.cpp

@@ -51,6 +51,11 @@ void wxPanel::DoSetBackgroundBitmap(const wxBitmap& bmp)
 {
     delete m_backgroundBrush;
     m_backgroundBrush = bmp.IsOk() ? new wxBrush(bmp) : NULL;
+
+    // Our transparent children should use our background if we have it,
+    // otherwise try to restore m_inheritBgCol to some reasonable value: true
+    // if we also have non-default background colour or false otherwise.
+    m_inheritBgCol = bmp.IsOk() || UseBgCol();
 }
 
 WXHBRUSH wxPanel::MSWGetCustomBgBrush()