Malia GUI camera change & resize

To reproduce this,

1. Open Malia in interactive mode with a scene
2. Maximize the window
3. Change the camera

Is that an expected behaviour?

Alternative possible behaviour:

• Have a flag _user_specified_window_size set to false when the scene is loaded
• Set this flag to true when the user resize the window
• When changing camera:
  • Change the window size if the flag is set to false
  • Change the sensor size to fit the viewport if the flag is set to true

A bit overkill though...