Raise ImportError instead of SystemExit when $DISPLAY is not set, so
regrtest.py will do the right thing.

Add a call to gl.clear() to clear the window before drawing in it.

Add some verbose prints for completeness.

Use gl.v2i() for the coordinates of one of the lines, for a little bit
of variety.

Reduce the time the window is displayed from 5 to 2 seconds.