This is an update to https://developer.blender.org/D583
This is essentially the same patchset as last year,
with the addition of two bugfixes (pixel_aspect handling
and a possible double free).
I have used this feature a lot on production stuff this year
and it's IMHO ready to get tested by a wider audience
Note anchor point I rebased against is c27589e19fc0 (Tag v2.75a)