added wxDIB::Save() and wxDIB to/from wxBitmap) conversions, use them instead of...

added wxDIB::Save() and wxDIB to/from wxBitmap) conversions, use them instead of the old C functions (the latter were removed, there is no old code left any more)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@19744 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775