Defined Rectangle in package.

- Instead of deriving from image.Rectangle.
Added methods on Rectangle.

rectangle.go

@@ -1,13 +1,48 @@
 package geom
 
-import (
-	_image "image"
-)
-
-// Rectangle is exposing the standard library image.Rectangle in the geom package.
-type Rectangle _image.Rectangle
+// Rectangle represents a 2-dimensional surface with as boundaries Min (inclusive) and Max (exclusive).
+type Rectangle struct {
+	Min, Max Point
+}
 
 // Rect is a shorthand function to create a rectangle.
 func Rect(x0, y0, x1, y1 int) Rectangle {
-	return Rectangle(_image.Rect(x0, y0, x1, y1))
+	if x0 > x1 {
+		x0, x1 = x1, x0
+	}
+	if y0 > y1 {
+		y0, y1 = y1, y0
+	}
+	return Rectangle{Point{x0, y0}, Point{x1, y1}}
+}
+
+// Add translates rectangle r by point p.
+func (r Rectangle) Add(p Point) Rectangle {
+	return Rectangle{
+		Point{r.Min.X + p.X, r.Min.Y + p.Y},
+		Point{r.Max.X + p.X, r.Max.Y + p.Y},
+	}
+}
+
+// Dx returns the width of r.
+func (r Rectangle) Dx() int {
+	return r.Max.X - r.Min.X
+}
+
+// Dy returns the height of r.
+func (r Rectangle) Dy() int {
+	return r.Max.Y - r.Min.Y
+}
+
+// Size returns the size of the rectangle.
+func (r Rectangle) Size() Point {
+	return Point{r.Max.X - r.Min.X, r.Max.Y - r.Min.Y}
+}
+
+// Sub translates rectangle r in the opposite direction of point p.
+func (r Rectangle) Sub(p Point) Rectangle {
+	return Rectangle{
+		Point{r.Min.X - p.X, r.Min.Y - p.Y},
+		Point{r.Max.X - p.X, r.Max.Y - p.Y},
+	}
 }