add high-level bindings for array pointers

add high-level bindings for array pointers

* figl/gl.scm (set-gl-vertex-array, set-gl-normal-array):
  (set-gl-color-array, set-gl-secondary-color-array):
  (set-gl-index-array, set-gl-fog-coordinate-array):
  (set-gl-texture-coordinates-array, set-gl-vertex-attribute-array):
  Add procedures for setting gl array pointers from structure data.

  (gl-enable-client-state, gl-disable-client-state):
  (gl-enable-vertex-attribute-array):
  (gl-disable-vertex-attribute-array):
  (set-gl-client-active-texture): Add procedures for enabling
  vertex arrays.

  (gl-array-element, gl-draw-arrays, gl-draw-elements): Add procedures
  for accessing the array data.

* figl/contrib/structures.scm (structure-field?): Export.