image: change a ColorImage's minimum point from (0, 0) to (-1e9, -1e9).

R=r
CC=golang-dev
https://golang.org/cl/2138045

src/pkg/image/names.go

@@ -29,7 +29,7 @@ func (c ColorImage) ColorModel() ColorModel {
 	return ColorModelFunc(func(Color) Color { return c.C })
 }
 
-func (c ColorImage) Bounds() Rectangle { return Rectangle{ZP, Point{1e9, 1e9}} }
+func (c ColorImage) Bounds() Rectangle { return Rectangle{Point{-1e9, -1e9}, Point{1e9, 1e9}} }
 
 func (c ColorImage) At(x, y int) Color { return c.C }