Fix transparent objects rendering

[SzymonOzog]

Motivation

Currently transparent objects are not rendered correctly and are highly dependant on the ordering of how the objects were added, consider the following example:

        l = Line3D(LEFT, RIGHT ,width=0.5).scale(2)
        l2 = Line3D(UP, DOWN, width=0.5, opacity=0.3).scale(2).move_to(l).shift(OUT)
        self.add(l2, l)

it renders like this:

If we switch the order in which we added the mobjects it starts rendering the opacity correctly

        l = Line3D(LEFT, RIGHT ,width=0.5).scale(2)
        l2 = Line3D(UP, DOWN, width=0.5, opacity=0.3).scale(2).move_to(l).shift(OUT)
        self.add(l, l2)

Proposed changes

We render fully opaqaue objects first and then transparent objects sorted by user specified z index as a primary key and then distance

Quoting from OpenGL FAQ

When using depth buffering in an application, you need to be careful about the order in which you render primitives. Fully opaque primitives need to be rendered first, followed by partially opaque primitives in back-to-front order. If you don't render primitives in this order, the primitives, which would otherwise be visible through a partially opaque primitive, might lose the depth test entirely.