3d window is flickering if multisampling is disabled

If you take the 500_triangle example with multisampling disabled

```
settings: {
    ...
    multisample: false
},
```

and disable clearing color buffer by changing

```
gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);
```

to

```
gl.clear(gl.DEPTH_BUFFER_BIT);
```

everything starts to flickr. Why? It's common technique for generative painting so I would expect it to work.
