[AD] preliminary ALLEGRO_USE

this patch should remove a lot of missing exports linker errors when linking together a library with mingw using C drawing code. please look over it. i'll admit i'm stuck at _do_stretch() however... are there any windows gurus on this list who can help? i'm wondering if we need to export *all* functions or if we just have to export those functions who must be call-able from outside the DLL. if so, that would solve these problems. for the moment dllwrap chokes if it finds that exported functions have been left out even if they are linked or not. Polygon scanline helper functions is a good example.