Fix #34040: Moving Normal Node with enabled Cycles Material Preview crashes

Issue was caused by couple of circumstances:

- Normal Map node requires tesselated faces to compute tangent space
- All temporary meshes needed for Cycles export were adding to G.main
- Undo pushes would temporary set meshes tessfaces to NULL
- Moving node will cause undo push and tree re-evaluate fr preview

All this leads to threading conflict between preview render and undo
system.

Solved it in  way that all temporary meshes are adding to that exact
Main which was passed to Cycles via BlendData. This required couple
of mechanic changes like adding extra parameter to *_add() functions
and adding some *_ex() functions to make it possible RNA adds objects
to Main passed to new() RNA function.

This was tricky to pass Main to RNA function and IMO that's not so
nice to pass main to function, so ended up with such decision:

- Object.to_mesh() will add temp mesh to G.main
- Added Main.meshes.new_from_object() which does the same as to_mesh,
  but adds temporary mesh to specified Main.

So now all temporary meshes needed for preview render would be added
to preview_main which does not conflict with undo pushes.

Viewport render shall not be an issue because object sync happens from
main thread in this case.

It could be some issues with final render, but that's not so much
likely to happen, so shall be fine.

Thanks to Brecht for review!

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54318 954f8c5b-7b00-dc11-b283-0030488c597c