Enable alpha test for textures with alpha transparency

Textures with alpha transparency currently displays the transparent pixels as black color. Enabling alpha test so that that they are transparent.

The changes are similar to what's done in this function in ign-rendering: https://github.com/ignitionrobotics/ign-rendering/blob/ign-rendering5/ogre2/src/Ogre2Material.cc#L161

which is being called here by ign-gazebo: https://github.com/ignitionrobotics/ign-gazebo/blob/ign-gazebo5/src/rendering/SceneManager.cc#L623

Here's a model for testing: https://staging-app.ignitionrobotics.org/iche033/fuel/models/Hello%20Box

Signed-off-by: Ian Chen ichen@osrfoundation.org