Calculation of the percentage of obscured

### Describe the issue

I want to get a target detection dataset for an industrial scene based on blenderproc rendering. In an industrial scene, there is a strong relationship between the degree of occlusion of an object and the capture priority. So I want to get the occlusion ratio of each object while rendering coco labels (occlusion ratio = object visible mask / object original mask). My current solution is to use bop_toolkit to compute the object's projection mask based on the target's bitmap, and then compute the target's visible mask, but this way I need to output the bitmap of each model when rendering as well, which is obviously inefficient. Do you have any good suggestions or ways to implement this please?

### Minimal code example

_No response_

### Files required to run the code

_No response_

### Expected behavior

Quickly get the proportion of each object that is occluded in the camera view after physical simulation

### BlenderProc version

v2.4.1

The issue names BlenderProc v2.4.1, COCO-label rendering, and bop_toolkit but no source file or test. Start by tracing the existing COCO annotation and mask-rendering entry points; done means exposing each object's occlusion ratio without requiring per-model bitmap output.

Written by the indexing model from the issue text.