stereo option in ViewerGui doesn't use the Settings framework

[pixelflinger]

This code

        StereoscopicOptions stereoOptions = mView->getStereoscopicOptions();
        ImGui::Checkbox("Instanced stereo", &stereoOptions.enabled);
        if (stereoOptions.enabled) {
            ImGui::SliderFloat("Ocular distance", &mOcularDistance, 0.0f, 10.0f);

        }
        mView->setStereoscopicOptions(stereoOptions);

in ViewerGui.cpp is wrong because it will activate stereo on the UI context (when running on Web with remote ui). Like with all other settings, stereo must use the Settings framework or be moved to the Debug tab. However, for testing it would be much better that we can use this on devices.