Webots allows you to display overlays from either cameras or displays. The camera overlay is not modifiable with the display api.

In order to draw on top of a camera feed, you can link a display to the camera and then draw on the display. To do this, create a Display node inside of your robot. You can then access it in your controller with robot.getDevice(). Then, you can make the display show the camera feed with display.attachCamera(). You should then be able to use the other display API functions to modify the image.

Note that you can toggle each overlay from the Overlays tab. Also check out the display sample world File > Open Sample World > samples > devices > display.wbt