Support multiple texture coordinate sets

[iche033]

The SubMesh class have always supported only texture coordinate sets per submesh. This works for most cases but when it comes do adding lightmaps to meshes, they usually occupy a different texture coordinate set, i.e. they are mapped onto the mesh in a different way than the typical albedo and normal maps. This pull request adds new APIs that enable users to add texture coordinates for different sets in the same submesh. I've also updated the ColladaLoader to load multiple texture coordinate sets.

[codecov]

Codecov Report

Merging #131 (1243429) into ign-common3 (3027dc0) will increase coverage by 0.11%.
The diff coverage is 89.61%.

@@               Coverage Diff               @@
##           ign-common3     #131      +/-   ##
===============================================
+ Coverage        75.38%   75.49%   +0.11%     
===============================================
  Files               69       69              
  Lines             9494     9608     +114     
===============================================
+ Hits              7157     7254      +97     
- Misses            2337     2354      +17     

Impacted Files	Coverage Δ
graphics/src/SubMesh.cc	91.31% <80.48%> (-4.14%)	⬇️
graphics/src/ColladaLoader.cc	85.17% <100.00%> (+0.49%)	⬆️

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[chapulina]

Nice! Lets get this in!