evas image: reload after alpha_set

evas image: reload after alpha_set

Summary:
The alpha_set cancels preload, and do not try to load it again.
So the image is not showing, if the alpha_set is called while preloading.

[Sample]
   Evas_Object *image = elm_image_add(box);
   elm_box_pack_end(box, image);
   evas_object_show(image);

   elm_image_file_set(image, "img.png", NULL);     // start preloading

   Evas_Object *eimg = elm_image_object_get(image);
   evas_object_image_alpha_set(eimg, EINA_TRUE);   // cancel preloading

Reviewers: Hermet, jsuya, herb

Reviewed By: Hermet

Subscribers: zmike, cedric, #reviewers, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D11864