valid_image_p doesn't load image libraries

[Juanma Barranquero <lekktu@gmail.com>]

On image dynamic-loading environments (AKA Windows), when the first
function to try to use an image type is a C primitive and not Lisp
code (for example, image-size), the corresponding image library is not
loaded.

The reason is that these functions call valid_image_p(), which in turn
calls lookup_image_type(), which does:

  if (NILP (Finit_image_library (symbol, Qnil)))
     return 0;

Now, init-image-library's second argument nil does not mean "use the
default library alist" (the variable `image-library-alist'), but "use
a null library alist", so the above call will always return nil unless
the library corresponding to the given image type is already loaded.

Now, assuming that lookup_image_type is the right place to load the
library, there's two possible answers:

  - passing `image-library-alist' to the call in lookup_image_type()
  - changing the spec of init-image-library so second arg nil means
"load from the default library alist".

Both of them require `image-library-alist', which is currently defined
on image.el, to be moved to C code.

Objections? Suggestions? Comments?

-- 
                    /L/e/k/t/u