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/+/171097Reviewed-by: Nigel Tao <nigeltao@golang.org>

src/image/geom.go

@@ -67,6 +67,8 @@ func (p Point) Eq(q Point) bool {
 }
 
 // ZP is the zero Point.
+//
+// Deprecated: Use a literal image.Point{} instead.
 var ZP Point
 
 // Pt is shorthand for Point{X, Y}.
@@ -254,6 +256,8 @@ func (r Rectangle) ColorModel() color.Model {
 }
 
 // ZR is the zero Rectangle.
+//
+// Deprecated: Use a literal image.Rectangle{} instead.
 var ZR Rectangle
 
 // Rect is shorthand for Rectangle{Pt(x0, y0), Pt(x1, y1)}. The returned