Patch by Zer0 (vince), many thanks! :)

(Hopefully!) Logic behind normals export indices was indeed wrong when using several objects (at least, not matching vertex/uv ones).

Update to OBJ exporter: now you can choose wether to export smooth groups IDs as simple values or as bitflags.

- r3954/patch [#32914], introduced incorrect displacement map name, corrected.
- added support for reading displacement maps.
- remove checks that added alpha support for diffuce images, OBJ has alpha material settings for this.

correct exporing of loose edges, obj spec defines faces as needing at least 3 verts (even though 2 vertex faces are in fact quite common).
now export these as lines.

remove use of ob.update_from_editmode() since editmode meshes don't get access to customdata layers. 

from dan grauer (kromar)

With change use_map_color_emission (which is for volumetrics only) to use_map_emit.

also ignore generated face-images (which have no file path).

Changed how mtl_mat names are generated, so that we can still be sure to always have a unique mtl_name for each key (mat_name, tex_name), yet avoiding to add tex name to mat when not needed.

bpy_extras.io_utils.path_reference() expects a real dir as base_src, not the blend file name!

update addons enabled by default to use 'faces -> tessfaces' also grease pencil scatter and quake map export.

from ed gautier (edddy)