Cherry pick OBJ fixes from current Assimp (3.x)

Fixes multiple issues, add consts,  better error output, and allow spaces in names

Change-Id: I9084ec7861ad0c71977c765da7c132ba75bbc46b
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Reviewed-by: James Turner <james.turner@kdab.com>