Tweaks to determination of slide show groups.
Tweaks to determination of slide show groups.

Slight adjustment to rules for defining static groups.
Adjustment of small bias towards 0,0 motion in first pass.

Change-Id: Id1d3753979ad54622f983f4de08472738317ec8e

ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx