Enable alpha based transparency on PBR materials by default

[luca-della-vedova]

This (simple) PR enables transparency on Physically Based Rendering materials. The alpha channel of the diffuse / albedo map is used to set the transparency.

An example way to test it is by using Pine Tree from Fuel, it will be necessary to update the model.sdf file by changing the <material> tag of the branch into:

        <material>
          <diffuse>1.0 1.0 1.0</diffuse>
          <script>
            <uri>model://Pine Tree/materials/scripts/</uri>
            <uri>model://Pine Tree/materials/textures/</uri>
            <name>PineTree/Branch</name>
          </script>
          <!-- PBR for Ignition, material scripts for gazebo classic-->
          <pbr>
            <metal>
              <albedo_map>model://Pine Tree/materials/textures/branch_2_diffuse.png</albedo_map>
            </metal>
          </pbr>
        </material>

Before and after the PR:

Open to feedback! (especially if this PR introduces any regression / change in behavior in preexisting assets / projects).

[codecov]

Codecov Report

Merging #249 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #249   +/-   ##
=======================================
  Coverage   65.78%   65.78%           
=======================================
  Files         128      128           
  Lines        6307     6307           
=======================================
  Hits         4149     4149           
  Misses       2158     2158           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 6a3f5dd...467cf64. Read the comment docs.

[iche033]

The pine tree model renders correctly for me now. I also tested with a few other assets and it seems to work fine!