image: deprecate ZP and ZR

They were added a very long time ago, as a convenience before Go had
struct literals. Today, it is better to use the zero-valued literal. For
example, the compiler cannot prove that ZP or ZR have not been modified.

Change-Id: I7469f1c751e91bf76fe1eab07b5772eccb5d6405
Reviewed-on: https://go-review.googlesource.com/c/go/+/171097


Reviewed-by: Nigel Tao <nigeltao@golang.org>