Fix handling of image cache refcounts. (Bug#20802)
This backports Eli Zaretskii's solution of this problem for W32
to X and NS.
* src/nsfns.m (image_cache_refcount): Define unconditionally.
(unwind_create_frame): If the image cache's reference count
hasn't been updated yet, do that now.
(Fx_create_frame): Set image_cache_refcount unconditionally.
* src/xfns.c (image_cache_refcount): Define unconditionally.
(unwind_create_frame): If the image cache's reference count
hasn't been updated yet, do that now.
(Fx_create_frame, x_create_tip_frame): Set image_cache_refcount
unconditionally.
* src/w32fns.c (image_cache_refcount): Make it a ptrdiff_t as on
X and NS.