evas: fix huge memory leak for non-async rendering

So... I had issues with evas-fb engine which was massively leaking,
one image per frame.
After investigating a bit with @cedric on IRC, the reference count
of the cache entries was always 2 before the engine dropped.
So, for each frame with an animation, we could never drop a cache
entry, leading to a trumendous amount of memory leaking.

Now for non-async rendering, we copy the behaviour of
evas_render_pipe_wakeup() which is called in async-mode,
and actually drops a reference in the cache entry.

Fixes T3763

src/lib/evas/canvas/evas_render.c

@@ -2949,6 +2949,8 @@ evas_render_updates_internal(Evas *eo_e,
         EINA_LIST_FOREACH(e->render.updates, l, ru)
           {
              post.updated_area = eina_list_append(post.updated_area, ru->area);
+             evas_cache_image_drop(ru->surface);
+             ru->surface = NULL;
           }
         eina_spinlock_take(&(e->render.lock));
         _cb_always_call(eo_e, EVAS_CALLBACK_RENDER_POST, post.updated_area ? &post : NULL);