OffscreenRender quality is very poor compared to Visualizer

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### Describe the issue

I'm am saving a screen capture of a textured 3D model and notice that the quality is very low and blurred using the `OffscreenRenderer` compared to the normal `Visualizer` when using the same resolution.

`Visualizer`

`OffscreenRenderer`

### Steps to reproduce the bug

*OffscreenRenderer Version*
```python
import os
os.environ['OPEN3D_CPU_RENDERING'] = "true"

import open3d as o3d
import open3d.visualization.rendering as rendering
import glob
import numpy as np

def main():
render = o3d.visualization.rendering.OffscreenRenderer(width=640, height=640)

# https://www.dropbox.com/s/4e3s2iae3jxih0h/palm.zip?dl=1
mesh = o3d.io.read_triangle_model(f'palm/model.obj')

render.scene.add_model(f'mesh', mesh)
cam = o3d.camera.PinholeCameraParameters()
cam.extrinsic = np.asarray([[1,0,0,0],[0,1,0,0],[0,0,1,150],[0,0,0,1]])
cam.intrinsic = o3d.camera.PinholeCameraIntrinsic(640, 640, 640, 640, 320, 320)
render.setup_camera(cam.intrinsic, cam.extrinsic)

# Save screenshot
image = render.render_to_image()
o3d.io.write_image("headless.png", image, 9)

# # Save screenshot. Maybe lights improve things? (Nope)
render.scene.scene.set_sun_light([0.0, 0.0, 0.0], [1.0, 1.0, 1.0],100000)
render.scene.scene.enable_sun_light(True)
image = render.render_to_image()
o3d.io.write_image("headless-lit.png", image, 9)

if __name__ == "__main__":
main()
```

*Visualizer Version*
```python
import numpy as np
import open3d as o3d
import cv2
if __name__ == "__main__":
# https://www.dropbox.com/s/4e3s2iae3jxih0h/palm.zip?dl=1
pcd = o3d.io.read_triangle_mesh(f'palm/model.obj', True)

vis = o3d.visualization.Visualizer()
vis.create_window(window_name="Open3D", width=640, height=640, visible=False)
opt = vis.get_render_option()
opt.point_size = 5.0
opt.background_color = [1, 1, 1]
opt.line_width = 1
vis.add_geometry(pcd)
vis.poll_events()
vis.update_renderer()

# Save screenshot
buf = vis.capture_screen_float_buffer()
buf = np.asarray(buf)
buf = (buf * 255.).astype('uint8')
buf = cv2.cvtColor(buf, cv2.COLOR_RGB2BGR)
cv2.imwrite('visualizer.png', buf)
vis.destroy_window()
```

### Error message

None.

### Expected behavior

Same quality screen capture.

### Open3D, Python and System information

```markdown
- Operating system: macOS 12.6
- Python version: Python 3.7
- Open3D version: `0.15.1` for Visualizer 0.16.1 for OffscreenRenderer
- System architecture: M1
- Is this a remote workstation?: no
- How did you install Open3D?: pip
```

The mesh used is available from the link in the comment.