added support for meshes with diffuse AND normal map

[charly-perspectives]

Hey there.

Following the issue #47, I modified obj2tiles in order tu support normal maps.

If you add the norm attribute in the .mtl file, the output 3DTiles will contain the normal map in its material.
Example:

newmtl default
Kd 0.5000000000 0.5000000000 0.5000000000
Ks 1.0000000000 1.0000000000 1.0000000000
Tr 0.0000000000
d 1.0000000000
Tf 1.0000000000 1.0000000000 1.0000000000
Pr 0.0000000000
Pm 0.0000000000
Pc 0.0000000000
Pcr 0.0000000000
Ni 1.5199999809
Ke 0.0000000000 0.0000000000 0.0000000000
illum 2
map_Kd textures\trap.png
norm textures\Box002NormalsMap.tga.png

• For some reason though, the normal map is showing only when there is also a diffuse map. I can't figure out why, as the normal map texture is indeed present in the 3D Tile. It might not even be related to Obj2Tile, but if you have an idea...
• The normal texture and diffuse have the be the same size for obj2tiles to function correctly.

Thank you for reading me, I'd like to know what do you expect for this PR to be mergeable.

[pierotofy]

Hi @charly-perspectives 👋 thanks for the PR (and sorry for the late review). This is certainly an interesting addition. I have no insights into the issue of needing both a diffuse and normal map, perhaps this is something glTF specific.