static GLenum get_gs_xfb_mode(GLenum mode)
{
   switch (mode) {
   case GL_POINTS:
      return GL_POINTS;
   case GL_LINE_STRIP:
      return GL_LINES;
   case GL_TRIANGLE_STRIP:
      return GL_TRIANGLES;
   default:
      fprintf(stderr, "illegal gs transform feedback mode %d\n", mode);
      return GL_POINTS;
   }
}

static GLenum get_gs_xfb_mode(GLenum mode)
{
   switch (mode) {
   case GL_POINTS:
      return GL_POINTS;
   case GL_LINE_STRIP:
      return GL_LINES;
   case GL_TRIANGLE_STRIP:
      return GL_TRIANGLES;
   default:
      fprintf(stderr, "illegal gs transform feedback mode %d\n", mode);
      return GL_POINTS;
   }
}
