Adding 'Col' vertex color layer to preview cycles meshes
The artist could simply disconnect the VCol node from the nodetree for
the preview, but it should be harmless to add this to the preview meshes.
As long as the user stick to the default vertex color name (and is happy
with seing a all-white channel) it should be ok.
This is needed because render result is being reused between different render
previews and keeping names out of sync means once preview result was created
for blender internal, it wouldn't be updated after switching to cycles.
Same goes to switching from cycles to other renderers like lux and yaf.
Simply updated cycles_preview.blend for now, could be solved from code side
as well, but IMO better to collect such a limitations of preview blend files
and solve them all together during "custom preview.blend for external engines"
project.
Reported by Jens in IRC.
Patch by Sergey, .blend by Thomas and some further tweaks by me.
Still to solve later: allow external engines to specify own preview .blend, for
now the code here is doing too much magic hacking on the preview scene still.
