evas: fix huge memory leak for non-async rendering

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