One is a more aggressive version of the pruned subpel tree
search where only a single halfpel candidate is searched.
The search candidate is based on a surface fit result.
The other is a method to obtain the subpel position at one
shot based on the same surface fit.

The methods have not been deployed in any speed setting yet.

Change-Id: I34fef3f2e34f11396c9d1ba97f4be8c4ffca62d3