The number of instance labels in the scene cannot exceed 255

### Describe the issue

I want to render the instance labels of a certain type of object in the scene, but when the number of objects exceeds 255, segmap can't save the labels of each instance because his encoding is int8, and it can only save the labels of 255 different instances at most.

### Minimal code example

```python
#load sampled objs...
bproc.renderer.enable_segmentation_output(map_by=['instance','class','name'])
#init light and camera..
data=bpro.renderer.render()
#save res...
```

### Files required to run the code

_No response_

### Expected behavior

Saving even when the number of instances exceeds 255

### BlenderProc version

V2.5.0