Cherry-pick #383 forward to master: Fix supported shader types (normal_map_X_space)

[scpeters]

Cherry-pick #383 forward to master, and I amended it to include the change in the sdf/1.8 folder. Use rebase and merge.

Original description:

Going over models on the Gazebo model database, I don't see a single model that uses:

• normal_map_objectspace
• normal_map_tangentspace

Instead, there are models using:

• normal_map_object_space
• normal_map_tangent_space

I suspect that the documentation has been wrong all along, and when the sdf::Material class was created, the error was propagated.

The fix here changes the documentation to the "correct" values, but keeps the "old" values supported to avoid breaking existing code.

[codecov-io]

Codecov Report

Merging #387 into master will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #387      +/-   ##
==========================================
- Coverage   87.40%   87.38%   -0.02%     
==========================================
  Files          60       60              
  Lines        9337     9341       +4     
==========================================
+ Hits         8161     8163       +2     
- Misses       1176     1178       +2     

Impacted Files	Coverage Δ
src/Material.cc	97.84% <100.00%> (-1.42%)	⬇️

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