Add speed feature to control global motion compute

Adds a speed feature to control which references to use
to compute global motion.
Also adds logic to not compute duplicate sets of
parameters when reference frames point to the same
buffers.
Includes some renaming of functions to set good speed
features to make things clearer.

Change-Id: I641d33441fde98af18cad8d4db49cf7d5d153ead