Large point cloud rendering not working

**Describe the bug**
Rendering large point clouds results in black output

**To Reproduce**
Steps to reproduce the behavior:
Running the following snipped shows this behavior, setting num_points to ~80 mio the rendering is fine, but with ~90mio+ it stops working.

```
import open3d as o3d
import numpy as np
import matplotlib.pyplot as plt

resolution = (1920, 1080)
num_points = 100000000
points = np.random.rand(num_points, 3)
pcd = o3d.geometry.PointCloud()
pcd.points = o3d.utility.Vector3dVector(points)
pcd.colors = o3d.utility.Vector3dVector(points)

mat = o3d.visualization.rendering.Material()
mat.shader = 'defaultUnlit'

renderer_pc = o3d.visualization.rendering.OffscreenRenderer(resolution[0], resolution[1])
renderer_pc.scene.set_background(np.array([0, 0, 0, 0]))
renderer_pc.scene.add_geometry("pcd", pcd, mat)

bounds = pcd.get_axis_aligned_bounding_box()
center = bounds.get_center()
max_dim = 1.25 * bounds.get_max_extent()
eye = (center[0], center[1], center[2] + max_dim)
up = (0, 1, 0)
renderer_pc.scene.camera.look_at(center, eye, up)

img = renderer_pc.render_to_image()
plt.imshow(img)
plt.show()
```

**Expected behavior**
I would expect it to render large point clouds or throw an error if this is not supported

**Screenshots**
80M points:

100M points:

**Environment (please complete the following information):**

- Operating system: Windows 10 64-bit
- Python version: Python 3.7
- Open3D version: 0.12
- Is this remote workstation?: no
- How did you install Open3D?: pip
- Compiler version (if built from source):