added experimental support for z_index

[Shimushushushu]

Test case

from manimlib import *

class TestCase(Scene):
    def construct(self):
        # circle1 = Circle()
        circle1 = Circle(z_index=1)
        circle1.set_fill(BLUE)
        circle1.set_stroke(BLUE_E, width=4)
        circle1.set_opacity(1)
        circle1.move_to(np.array([0.5, 0, 0]))

        # circle2 = Circle()
        circle2 = Circle(z_index=0)
        circle2.set_fill(RED)
        circle2.set_stroke(RED_E, width=4)        
        circle2.set_opacity(1)
        circle2.move_to(np.array([-0.5, 0, 0]))

        self.add(circle1)
        self.add(circle2)