Fix GDI+ image loading by file name

Fix GDI+ image loading by file name

Without a call to image_find_image, we can get a file name that
is relative to data-directory/images/, or a file name that
starts with "~/", in which case w32_load_image would fail.
* src/image.c (native_image_load): Call image_find_image_file to
resolve and encode the image file name.
* src/w32image.c (w32_load_image): No need to encode the file
name, as it's already encoded by native_image_load.