Blender Git Loki

Refactor readfile's liblink code.

Liblink specific ID type function was so far running a loop over all IDs
of relevant type, unlike almost any other 'ID-callback-like' functions
in Blender, which usually let the looping controll to calling code.

The latter approach is more convinient when one want to add generic
(i.e. type-agnostic) code, since it typically only has to change code in
one place (caller function) instead of tens of places (all the callback
functions).

This commit also changes/sanitizes a few things that had nothing to do
in main liblink code, like mesh conversion from tessfaces to polys
(which can be done in after-linking versionning code), or scenes' cycles
detection/check regarding background 'set' scenes.

Differential Revision: https://developer.blender.org/D6727