* Validate OBJ Texture-Coordinate Indices
load_obj read the texture coordinates of a face without checking the
vt index, so a face referencing a vt past the end of the list read out
of bounds and crashed, and a face vertex with no vt read index -1.
Out-of-range or missing indices now fall back to a zero UV. The face
keeps its entry in the per-face UV list, so the following faces stay
aligned, and the geometry loads as before.
Negative (relative) vt indices were also rebased by dividing the float
count by 3, but each vt stores two floats.
* Reject DRC Meshes Without Positions or with Invalid Face Indices
load_drc dereferenced the POSITION attribute without checking that the
mesh has one, and trusted the decoded face indices, which the Draco
decoder does not check against the point count. Both now fail the load
cleanly. A failed vertex conversion is treated the same way.
The libslic3r tests link Draco so they can encode the malformed meshes
in-test.
* Keep OBJ Texture Coordinates That Carry a W Component
The vt parser stopped reading the optional third component when texture
coordinates were cut down to u and v, but the check that nothing is left
on the line stayed. A legal "vt u v w" line was therefore rejected and
silently dropped, shifting every later texture index. The w component is
parsed again and discarded.
The texture coordinate stride is now a named constant, OBJ_TEXCOORD_LENGTH,
used by the parser and the importer, so the relative-index rebase cannot
drift from the storage layout again.
