Added RectRel{,F,F32} methods.

- Instead of a second coordinate (x,y) you can provide the width & height of the rectangle.

rectangle.go

@@ -16,6 +16,11 @@ func Rect(x0, y0, x1, y1 int) Rectangle {
 	return Rectangle{Point{x0, y0}, Point{x1, y1}}
 }
 
+// RectRel is a shorthand function to create a rectangle specifying its width and height instead of a second coordinate.
+func RectRel(x, y, w, h int) Rectangle {
+	return Rectangle{Point{x, y}, Point{x + w, y + h}}
+}
+
 // Add translates rectangle r by point p.
 func (r Rectangle) Add(p Point) Rectangle {
 	return Rectangle{

rectanglef.go

@@ -17,6 +17,11 @@ func RectF(x0, y0, x1, y1 float64) RectangleF {
 	return RectangleF{PtF(x0, y0), PtF(x1, y1)}
 }
 
+// RectRelF is a shorthand function to create a rectangle specifying its width and height instead of a second coordinate.
+func RectRelF(x, y, w, h float64) RectangleF {
+	return RectangleF{PointF{x, y}, PointF{x + w, y + h}}
+}
+
 // Add translates rectangle r by point p.
 func (r RectangleF) Add(p PointF) RectangleF {
 	return RectangleF{

rectanglef32.go

@@ -17,6 +17,11 @@ func RectF32(x0, y0, x1, y1 float32) RectangleF32 {
 	return RectangleF32{PtF32(x0, y0), PtF32(x1, y1)}
 }
 
+// RectRelF32 is a shorthand function to create a rectangle specifying its width and height instead of a second coordinate.
+func RectRelF32(x, y, w, h float32) RectangleF32 {
+	return RectangleF32{PointF32{x, y}, PointF32{x + w, y + h}}
+}
+
 // Add translates rectangle r by point p.
 func (r RectangleF32) Add(p PointF32) RectangleF32 {
 	return RectangleF32{