Voxel Depth-Map Carving removes everything behind the camera

**Describe the bug**
When ever your camera is positioned within a voxel grid anything "behind" the camera taking into accounts its intrinsic parameters is also carved when using voxelgrid.carve_depth_map.

**To Reproduce**
Carve a depth map with the camera positioned in the middle of the voxel grid. You will get two pyramids one front facing and one rear facing.

**Expected behavior**
Its not defined but it would be nice if things behind the camera were untouched as this allows for carving out an internal space such a room scene or other scenario where the camera position remains within the space and is pointed out towards the area of interest.

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

- Operating system: OSX 10.15
- Python version: (e.g. Python 3.6) 3.9
- Open3D version: 0.13