Blender Git Loki

Make planar tracking much faster.

- This makes planar tracking around 2-3x or more faster than
before, by rearranging how the sampling is done.
Previously, the source patch was sampled repeatedly on
every optimizer iteration; this was done for
implementation speed, but was wasteful in computation.

- This also contains some additions to Ceres to help
deailing with mixed numeric / automatic differentation. In
particular, there is now a "Chain::Rule" operator that
facilitates calling a function that takes Jet arguments,
yet does numeric derivatives internally. This is used to
mix the numeric differentation of the images with the warp
parameters, passed as jets by Ceres to the warp functor.

There is also a new "JetOps" object for doing operations
on types which may or may not be jets, such as scaling
the derivative part only, or extracting the scalar part
of a jet.

The Ceres patches are aimed at upstream.

- A new function for sampling a patch is now part of the
track_region.h API; this will get used to make the preview
widget properly show what is getting tracked. Currently
the preview widget does not handle perspective tracks.

Known issues:

This patch introduces a bug such that the "Minimum
Correlation" flag does not work; if it is enabled, tracking
aborts immediately. The workaround for now is to disable the
correlation checking, and examine your tracks carefully. A
fix will get added shortly.