Fix T45234: Stereo Parallel vs. Off-Axis

Parallel rendering was not working.

The idea of having parallel convergence mode to render as parallel but
visualize as off-axis was good, but it was leading to some complications
in the code.

I think it's more clear to the user if parallel looks and render as
parallel, and if she wants to pre-visualize the converged planes, simply
temporarily set the camera to off-axis.

release/scripts/startup/bl_ui/properties_data_camera.py

@@ -153,10 +153,9 @@ class DATA_PT_camera_stereoscopy(CameraButtonsPanel, Panel):
         col = layout.column()
         col.row().prop(st, "convergence_mode", expand=True)
 
-        if st.convergence_mode == 'PARALLEL':
-            col.prop(st, "viewport_convergence")
-        else:
-            col.prop(st, "convergence_distance")
+        sub = col.column()
+        sub.active = st.convergence_mode == 'OFFAXIS'
+        sub.prop(st, "convergence_distance")
 
         col.prop(st, "interocular_distance")