The viewport stereoscopy support helpers are finally ported to 2.80.
We now can scale the camera and the "stereo cameras" will scale
in the viewport as well (unlike 2.7x).
At the moment I disabled the drawing of the camera frame when
stereo is selected and you are looking through the camera.
It is to be fixed later, but for now it draws the border wrong.
In 2.79 this was not a problem because the camera frame was drawn
afterwards as a hack.
Viewport > Stereoscopy:
* Cameras
* Convergence plane
* Convergence plane alpha
* Stereoscopy volume
* Stereoscopy volume alpha
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