Ogre2 transparency issue

**Original report ([archived issue](https://osrf-migration.github.io/ignition-gh-pages/#!/ignitionrobotics/ign-rendering/issues/60)) by Ian Chen (Bitbucket: [Ian Chen](https://bitbucket.org/%7Beaa6fca5-6deb-43f6-907f-971c144735dd%7D/), GitHub: [iche033](https://github.com/iche033)).**

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When a visual is set to be transparent, sometimes the object behind the transparent object disappears / becomes as the camera pans from around it.

see gif in this comment: [https://osrf-migration.github.io/ignition-gh-pages/#!/ignitionrobotics/ign-gazebo/pull-requests/547/add-support-for-setting-visual/diff#comment-140398963](https://osrf-migration.github.io/ignition-gh-pages/#!/ignitionrobotics/ign-gazebo/pull-requests/547/add-support-for-setting-visual/diff#comment-140398963)

> I had a chance to play with it with the Cafe model \(chosen since it’s quite a complex one\) and observed some visual glitches.
>
> … but you can see an object disappearing after the first window and the walls becoming white after the second.

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