[Grace-core] Simple graphics and GUI interface

[Andrew P. Black]

> 
> But here you're suggesting using a "point" to represent something that
> is just a pair of numbers. Width and height are not a point. Opposite
> corner is a point.

The width and height of a rectangle — it’s extent — can be described by a 2D Vector — which is what we are calling Point2D.   It’s convenient to use a vector rather than a pair of numbers because you can then do arithmetic on them, such as

	def corner = origin + extent
	def center = origin + (extent / 2)

These formulae are simpler to both write and to read than 

	def cornerX = origin.x + width
	def cornerY = origin.y + height
	def corner = cornerX at cornerY

		Andrew