Simplify instancing creation

- Currently to create instances for particles or any other use, you have to make a large set of steps like:
- calculate the number of particles based on workgroup size and thread size
- pass those values (number of particles, worgroup size per dimension and threads per dimension) as constants to the library
- use those constants in the compute pass to calculate the @workgroup_size
- to indicate the number of instances you have to tell it to the added mesh this way with the `instanceCount`

```
glb_renderpass
.addMesh('lucy', positions, colors, colorSize, uvs, normals, indices)
.instanceCount = NUMPARTICLES;
```

It would be nice to have all of this in one step, like

renderPass.addInstances(meshName, numInstances)