evas image: fix a bug in image preloading.

evas image: fix a bug in image preloading.

Summary:
Current preloading is too buggy since it's on thread-based.
This is a fundamental improvement to fix a bug.

The critical issue here is,
When preloading img object suddenly cancel its preloading,
the object possibly cannot render image next then because
renderer doesn't have any idea when async cancelling is
finished. Renderer just tries to render regardless of
image loading status, and this could occur no-texture(in gl case)
image object.

So, here improvement is, adding a notification for async cancelled
so that putting img objects to redraw images properly after their preloading is
cancelled.

The best scenario to reproduce this bug is this one.

Evas_Object *img2 = evas_object_image_filled_add(evas);
evas_object_image_file_set(img2, "test.jpg", NULL);
evas_object_image_preload(img2, EINA_FALSE);
evas_object_resize(img2, 200, 200);
evas_object_show(img2);

Evas_Object *img = evas_object_image_filled_add(evas);
evas_object_image_file_set(img, "test.jpg", NULL);
evas_object_image_preload(img, EINA_FALSE);
evas_object_move(img, 200, 200);
evas_object_resize(img, 200, 200);
evas_object_show(img);

evas_object_image_preload(img2, EINA_TRUE);
If you run this on gl backend, occasionally happens rendering fail.

Yet there other bugs on preloading feature....

@fix

Reviewers: #committers, raster

Subscribers: cedric, #reviewers, #committers, zmike

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D6919