Assignment #1:

This assignment will give you some practice using simple OpenGL event
based programming. Also, you should become familiar with structs and
how to use them.

In this program you will create a list/vector of line segments
produced by the user through both keyboard and mouse input.

Mouse: 
   LEFT DOWN: The user will click and hold the left mouse button to start a
     line segment. 
   LEFT UP: The user will let go of the left mouse button to end a
     line segment.

Keyboard: 
   's': The user will press 's' to start a line segment. 
   'e': The user will press 'e' to end a line segment.
   'p': The user will press 'p' to print all line segments in the system.


Things that will help:
  Create a struct to hold the x and y positions of a point. 
  Create a struct to hold a line (which is two points).
  Create a vector of lines as a global variable. 
     (We'll discuss better usage of globals later in this course.)
  Don't forget to push_back lines that have an end into the vector.