Fix Bug #577; By rewriting OpenDIB in terms of wxImage/wxBitmap it can now read top-down bitmaps.
It still cannot handle 16bpp top-down bitmaps, which may be a bug in wxWidgets.
It also now leaks memory when loading malformed bitmaps. This may also be a bug in wxWidgets.
In addition to fixing this bug, using wxWidgets should enable bitmaps to be loaded on GNU/Linux (GTK+).