Introduces GZ_MESH_FORCE_ASSIMP flag

[onurtore]

These flags utilized to use assimp for mesh loading process. If useAssimp flag is true then STL, GLTF and FBX meshes are loaded using assimp library, otherwise old loaders will be used. forceAssimp flags used to load all supported mesh formats using assimp.

[onurtore]

@chapulina,
What do you think?

[chapulina]

Thanks! Could you document the new environment variables? Maybe on the MeshManager's header?

[chapulina]

Looks great, thanks! Since we're not checking extensions when forcing assimp, it would be nice to add a test where GZ_MESH_FORCE_ASSIMP=true and we try to load a mesh format that isn't supported by assimp (i.e. mesh.invalid), to make sure that doesn't cause a crash.

[codecov]

Codecov Report

Merging #403 (ee6badb) into luca/assimp_sandbox (4484e5e) will decrease coverage by 0.00%.
The diff coverage is 80.00%.

@@                   Coverage Diff                   @@
##           luca/assimp_sandbox     #403      +/-   ##
=======================================================
- Coverage                80.35%   80.34%   -0.01%     
=======================================================
  Files                       85       85              
  Lines                     9933     9944      +11     
=======================================================
+ Hits                      7982     7990       +8     
- Misses                    1951     1954       +3     

Impacted Files	Coverage Δ
graphics/src/MeshManager.cc	91.82% <80.00%> (-0.28%)	⬇️

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[onurtore]

Looks great, thanks! Since we're not checking extensions when forcing assimp, it would be nice to add a test where GZ_MESH_FORCE_ASSIMP=true and we try to load a mesh format that isn't supported by assimp (i.e. mesh.invalid), to make sure that doesn't cause a crash.

Hi @chapulina,
I think IsValidFilename function in MeshManager checks regardless of the mesh loader, and assimp loader covers the previous formats as well, so currently there is no unsupported format for the assimp loader.

On the other hand, I am very open to ideas about possible test cases, so if you have anything in mind, I would love to implement it.

[chapulina]

I added a simple test in a1c0b9f as part of #393, that loads a non-supported extension and verifies that a nullptr is returned. That's all I had in mind 😄