(x_create_x_image_and_pixmap) [HAVE_NTGUI]: Cast 4th arg

(x_create_x_image_and_pixmap) [HAVE_NTGUI]: Cast 4th arg
to CreateDIBSection to avoid a compiler warning.
(pbm_load): Cast 3rd arg to IMAGE_BACKGROUND to avoid a compiler
warning.
(png_load): Cast return values of fn_png_create_read_struct and
fn_png_create_info_struct, to avoid compiler warnings on W32.
Cast 3rd arg to IMAGE_BACKGROUND and image_background_transparent
to avoid compiler warnings.
(jpeg_load): Cast return value of fn_jpeg_std_error to avoid a
compiler warning on W32.  Cast 3rd arg to IMAGE_BACKGROUND to
avoid a compiler warning.
(tiff_load): Cast return values of fn_TIFFOpen and
fn_TIFFClientOpen to avoid compiler warning on W32.  Cast 3rd arg
to IMAGE_BACKGROUND to avoid a compiler warning.
(gif_load): Cast return values of fn_DGifOpenFileName and
fn_DGifOpen to avoid compiler warnings on W32.  Cast 3rd arg to
IMAGE_BACKGROUND to avoid a compiler warning.
(DrawText) [HAVE_NTGUI || MAC_OS]: If already defined, undef
before redefining.