Add win_posx win_posy params to display_paramts_t

In an effort to encapsulate glut window display parameters in the
display_params_t, I've started the process of coding simple OpenGL
routines, and noticing the params/values I've missed.

The first change to come out of this is the addition of the win_posx
and win_posy parameters, which tell glut where to display the top left
corner of the new window.