image-cache-size improvements

image-cache-size improvements

Implement for non-Cairo X11 and NS.  Count masks as well, and
XImage objects on X11.

* src/image.c (image_size_in_bytes): New.
(image_frame_cache_size): Use image_size_in_bytes.
* src/nsterm.h:
* src/nsimage.m (ns_image_size_in_bytes, [EmacsImage sizeInBytes]):
New function and method.
* src/w32gui.h:
* src/w32term.c (w32_image_size): Update signature.