evas_image: do not render while preloading

Summary:
Unexpected image shows if image data is not ready.
Even though there is a change to check the 'preloading' in pre_render phase,
evas_object_image_render is called. So we need to check here as well.

Reference: https://phab.enlightenment.org/D6739

It seems that the 'preloading' is not enough. The 'preloading' could be reset
to FALSE by _evas_image_load_async_cancel > _image_preload_internal.

If the following step happens, then this patch set is neccessary.
(1) evas_object_image_pre_render
(2) _evas_iamge_load_async_start
(3) evas_object_image_render

I could not find out what the correct step, but it actullay happens.
The evas_object_image_render could be called with the 'preloading' TURE.

Reviewers: Hermet

Reviewed By: Hermet

Subscribers: cedric, #reviewers, #committers, zmike

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D6778

src/lib/evas/canvas/evas_object_image.c

@@ -1749,6 +1749,9 @@ evas_object_image_render(Evas_Object *eo_obj, Evas_Object_Protected_Data *obj, v
 {
    Evas_Image_Data *o = type_private_data;
 
+   /* image is not ready yet, skip rendering. Leave it to next frame */
+   if (o->preloading) return;
+
    if ((o->cur->fill.w < 1) || (o->cur->fill.h < 1))
      return;  /* no error message, already printed in pre_render */