Add global motion experiment to rdopt

This patch completes the global motion experiment
implementation. It modifies the format of the motion
parameters to use the mv union to facilitate faster
copying and checks for parameters equal to 0 that occur
frequently in rdopt. The rd decisions for the global motion experiment
have also been added to rdopt.
Change-Id: Idfb9f0c6d23e538221763881099c5a2a3891f5a9