Fix several correctness issues where the new OBJ/MTL importer was not
producing the same results as the old one, mostly because the code for
some reason had slightly different logic. Fixes T97757:
- When .obj file tries to use a material that does not exist, the code
was continuing to use the previous material, instead of creating new
default one, as the previous importer did.
- Previous importer was always searching/parsing "foo.mtl" for a
"foo.obj" file, even if the file itself does not contain
"mtllib foo.mtl" statement. One file from T97757 repros happens to
depend on that, so resurrect that behavior.
- When IOR (Ni) or Alpha (d) are not specified in .mtl file, do not
wrongly set -1 values to the blender material.
- When base (Kd) or emissive (Ke) colors are not specified in the .mtl
file, do not set them on the blender material.
- Roughness and metallic values used by viewport shading were not set
onto blender material.
- The logic for when metallic was set to zero was incorrect; it should
be set to zero when "not using reflection", not when "mtl file does
not contain metallic".
- Do not produce a warning when illum value is not spelled out in .mtl
file, treat as default (1).
- Parse illum as a float just like python importer does, as to not
reintroduce part of T60135.
Reviewed By: Howard Trickey
Differential Revision: https://developer.blender.org/D14822
5f5e7ac317