Rework how GDI+ functions are loaded dynamically in w32image.c

Rework how GDI+ functions are loaded dynamically in w32image.c

* src/w32image.c: Define correct WINGDIPAPI typedefs for GDI+
functions.  We cannot use DEF_DLL_FN, since that is for functions
with C calling conventions, whereas GDI+ functions are __stdcall.
(gdiplus_init): Load functions from DLL manually, not via
LOAD_DLL_FN, as the latter is for __cdecl functions.
(w32_frame_delay): Initialize delay with a negative value, as zero
is a valid delay.