This is the ussual kind of OpenGL viewer for height fields, such as those you
generate in assignment 5. It's compiled the usual way, and run as:

   ./hfviewer <input_directory> [num_tiles]

It reads in height field files in the given directory, starting at heightfield0000.
If you supply an extra integer argument n, it draws an n*n grid of the height field
tiles instead of just one.

Clicking and dragging the mouse changes the view in a reasonably
intuitive manner.

Keyboard commands are:
  f  toggle full-screen mode
  q  quit
  s  save image (in hw5_0000.ppm, hw5_0001.ppm, ...)
  w  toggle wire-frame view
