cffi warnings about struct types when using GL-ARRAYs

[3b]

for example running (cl-glut-examples:misc-opengl-array) prints a bunch of warnings like

STYLE-WARNING:
   bare references to struct types are deprecated. Please use (:POINTER
                                                               (:STRUCT
                                                                CL-GLUT-EXAMPLES::POSITION-COLOR)) or (:STRUCT
                                                                                                       CL-GLUT-EXAMPLES::POSITION-COLOR) instead.

[3b]

see also issue #27.