[Bf-codereview] Tiles Compositing Branch (issue 6199066)

[brechtvanlommel at gmail.com]

On the Blender integration/UI level, these are all the comments I have
and bugs I found. I didn't look into the code for all nodes and
operations and test them, don't really have time to verify them all,
from what I gather users have been testing this quite well with many
reports in the tile tracker fixed.

Further, I added a hidden switch to use the old compositor in the branch
now. Eventually that should of course be removed, along with most of the
old node execution code, but for now it's nice to test.

http://codereview.appspot.com/6199066/