Fix converting wxBitmap to wxImage (wxGTK2)

Since f7247086c2 ("Fix storing wxBitmap data in GdkPixbuf", 2019-09-18),
919a4ec702 ("Fix drawing wxBitmap with mask", 2019-09-18) and other
commits (see #18498, #18508) RGBA wxBitmaps with masks are drawn properly
under wxGTK2 so only wxBitmap raw RGBA data should be transferred to
wxImage RGBA data because mask is stored in the target wxImage separately.

src/gtk/bitmap.cpp

@@ -836,7 +836,7 @@ wxImage wxBitmap::ConvertToImage() const
     // prefer pixbuf if available, it will preserve alpha and should be quicker
     if (HasPixbuf())
     {
-        GdkPixbuf *pixbuf = GetPixbuf();
+        GdkPixbuf *pixbuf = GetPixbufNoMask();
         unsigned char* alpha = NULL;
         if (gdk_pixbuf_get_has_alpha(pixbuf))
         {