You are given an m x n rectangular grid and want to find a spiral that visits
every point.  For example, here's a spiral on a 4x3 grid:

	*--*--*--*
	         |
	*--*--*  *
	|        |
	*--*--*--*

Write a function that takes in m and n and returns the coordinates of the vertices
of the spiral in the order you visit them.  Be as efficient as possible.