FadeOut and transparency

When fading out a composed Mobject (i. e. containing submobs), the pixels fade to black instead of the color of the mobject hidden behind it.

```
from helpers import *
from mobject import Mobject
from mobject.vectorized_mobject import *
from animation.animation import Animation
from animation.transform import *
from animation.simple_animations import *
from topics.geometry import *
from scene import Scene
from camera import Camera

class SampleScene(Scene):

def construct(self):

lines = Mobject() # VMobject()
line1 = Line(LEFT, RIGHT)
line2 = Line(UP, DOWN)
lines.add(line1, line2)

rect = Rectangle(fill_color = BLUE, fill_opacity = 1)

self.add(lines)
self.add(rect)

self.wait()

self.play(FadeOut(lines))
```

Result:

This does not happen in the exported PNG, and also not if `lines` is instantiated as a `VMobject`.