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

this is an internal name only.

T35358: Fix for addons using raise without Exception object. TODO: Find better way to terminate addons and inform user. More py 2.x leftovers in startup/bl_operators/uvcalc_*!

(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.

use preference for relative paths.

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

make addons blender versions consistent

don't overwrite alpha with material `illum` setting, or when using an alpha texture- if its already been set.

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).

use Tr as translucency and use reflection coords for 'map_refl' setting.