code: def canvas2pix(self, pos):
          
          x, y, z = pos
          #print('c2p in', x, y, z)
          mm = gl.glGetDoublev(gl.GL_MODELVIEW_MATRIX)
          pm = gl.glGetDoublev(gl.GL_PROJECTION_MATRIX)
          vp = gl.glGetIntegerv(gl.GL_VIEWPORT)
  
          pt = glu.gluProject(x, y, z, mm, pm, vp)
          #print('c2p out', pt)
          return pt
docs: Takes a 3-tuple of (cx, cy, cz) in canvas coordinates and gives
          the (x, y, z) pixel coordinates in the window.