vp9-svc: Define rc scene change flag per superframe

vp9-svc: Define rc scene change flag per superframe

Define the rc->high_num_blocks_with_motion, set in the
scene change analysis, to be defined per superframe.
This is used for increasing motion search area on
some (super)frames, e.g., for scrolling.

Also some code cleanup in rt_speed_feature_.

No change in behavior.

Change-Id: I1a5c04b9cd4aef1723ce42f82e981a2ca15c8b9d