go.mobile/geom: introduce Rectangle

LGTM=adg
R=adg, nigeltao
CC=golang-codereviews
https://golang.org/cl/151820043

geom/geom.go

@@ -17,6 +17,7 @@ type Pt float32
 // Px converts the length to current device pixels.
 func (p Pt) Px() float32 { return float32(p) * Scale }
 
+// String returns a string representation of p like "3.2pt".
 func (p Pt) String() string { return fmt.Sprintf("%.2fpt", p) }
 
 // Point is a point in a two-dimensional plane.
@@ -24,7 +25,17 @@ type Point struct {
 	X, Y Pt
 }
 
-func (p Point) String() string { return fmt.Sprintf("Point(%.2f, %.2f)", p.X, p.Y) }
+// String returns a string representation of p like "(1.2,3.4)".
+func (p Point) String() string { return fmt.Sprintf("(%.2f,%.2f)", p.X, p.Y) }
+
+// A Rectangle is region of points.
+// The top-left point is Min, and the bottom-right point is Max.
+type Rectangle struct {
+	Min, Max Point
+}
+
+// String returns a string representation of r like "(3,4)-(6,5)".
+func (r Rectangle) String() string { return r.Min.String() + "-" + r.Max.String() }
 
 // Scale is the number of pixels in a single Pt on the current device.
 //