Fix obj file loading

[NarinderS]

Fixes #83.

When loading obj files, the attrib.normals vector only contains the unique normals present in the mesh. To associate a normal to a vertex you have to use the face indices structure. In the current code, the following obj file causes an out of bounds array access:

o Plane
v 0.000000 0.000000 0.000000
v 0.353553 0.353553 0.000000
v 0.353553 -0.353553 0.000000
vn -0.0000 -0.0000 1.0000
s 0
f 2//1 1//1 3//1

I think this issue is present with UV coordinate loading as well.

[NarinderS]

One thing missing from this is how to deal with vertices with multiple normals, for example:

o Plane
v 0.000000 0.000000 0.000000
v 0.353553 0.353553 0.000000
v 0.353553 -0.353553 0.000000
v 0.044583 -0.475301 0.307226
vn -0.0000 -0.0000 1.0000
vn 0.5022 0.5022 0.7040
s 0
f 2//1 1//1 3//1
f 1//2 4//2 3//2

Here vertex 1 and 3 have two normals, and I think only face or averaged normals make sense here?

[fwilliams]

Looks like this would fail for uv coords and colors as well -- can we fix this and add a small test with the file you uploaded?

[NarinderS]

I don't think vertex colors will be a problem, since the OBJ file format forces every vertex to redefine the color, even if it's not unique. Unlike vertex normals and UV, which can be reused.

[fwilliams]

Good point about colors. We should still make the same change for texcoords at the line below though

[fwilliams]

Thanks for the pull request and sorry for getting to it so late -- I've had less time to focus on open source lately. I added one small comment above then this should be mergeable.

[fwilliams]

Looks great! Thanks for the fix! I'll merge when the CI passes