draw grid

def grid(size=1, n=10, width=1):
    """draw a grid on xz plane"""
    gl.glLineWidth(width)
    s = size / float(n)
    s2 = 0.5 * size
    batch = pyglet.graphics.Batch()

    for i in range(0, n + 1):
        x = -s2 + i * s
        batch.add(2, gl.GL_LINES, None, ('v3f', (x, 0, -s2, x, 0, s2)))
    for i in range(0, n + 1):
        z = -s2 + i * s
        batch.add(2, gl.GL_LINES, None, ('v3f', (-s2, 0, z, s2, 0, z)))

    batch.draw()