Resolve "(viz): 3D scene aspect ratio is broken in some apps"

Description

This fixes a minor bug in the initialisation phase of the renderer. Depending on the order of the events, we could access to a not yet initialized camera and base the aspect ratio computation on it... Which would lead to a division by zero.

Closes #1098 (closed)

How to test it?

Run the toolCalibration activity of SightCalibrator, as described in #1098 (closed)