Fix transparency issue for textures without alpha channel

[iche033]

When SetAlphaFromTexture(true) is invoked on a material with texture that does not have an alpha channel, it causes the visual with the assigned material to be completely transparent. The fix is to do a check to make sure the texture does have an alpha channel, and if not we disable using alpha from the texture.

These changes were included in pull request #182 for ign-rendering5, which are necessary for displaying the Indoor Lighting model.

Another test: launch ign-gazebo and insert the DronePlatformX1 model. Without these changes, the model is transparent.

[nkoenig]

Works great.

[codecov]

Codecov Report

Merging #186 (cc5c4a6) into ign-rendering4 (26497e7) will decrease coverage by 0.00%.
The diff coverage is 71.42%.

@@                Coverage Diff                 @@
##           ign-rendering4     #186      +/-   ##
==================================================
- Coverage           52.58%   52.58%   -0.01%     
==================================================
  Files                 143      143              
  Lines               13333    13337       +4     
==================================================
+ Hits                 7011     7013       +2     
- Misses               6322     6324       +2     

Impacted Files	Coverage Δ
ogre2/src/Ogre2Material.cc	94.75% <71.42%> (-0.69%)	⬇️

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[iche033]

@osrf-jenkins run tests please