Improves and simplifies costing for global motion to use the
actual bits used to communicate the global motion parameters.
Removes some of the old hacks.
This patch also includes necessary refactoring to incorporate
reference based coding of the motion parameters to follow.

lowres results: -1.489% (up from -1.333%)

Change-Id: I994dc97046011de4261633ccb0c9d71e374f2c5a