Fix alpha channel values when using wxGCDC with wxMemoryDC in wxMSW.

Ensure that 32bpp bitmaps selected in wxMemoryDC use DIB for their internal
representation as GDI+ functions don't seem to work correctly with DDBs with
alpha channel.

Closes #13328.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75648 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/bitmap.h

@@ -136,6 +136,8 @@ public:
 #if wxUSE_WXDIB
     // copies from a device independent bitmap
     bool CopyFromDIB(const wxDIB& dib);
+    bool IsDIB() const;
+    bool ConvertToDIB();
 #endif
 
     virtual bool Create(int width, int height, int depth = wxBITMAP_SCREEN_DEPTH);