Fixes T97794 (which is a reintroduction of an older issue T67266 that
has been fixed in the python importer, but the fix was not in the C++
one). Some software produces OBJ files with mtllib statements like
mtllib "file name in quotes.mtl", and the new importer was not stripping
the quotes away.
While at it, I noticed that MTLParser constructor was taking a StringRef
and treating it as a zero-terminated string, which is not necessarily
the case. Fixed that by explicitly using a StringRefNull type.
Reviewed By: Howard Trickey
Differential Revision: https://developer.blender.org/D14838
cbeb8770cc