vp9: Changes for scene detection overshoot and SVC.

vp9: Changes for scene detection overshoot and SVC.

Refactor the scene detection for 1 pass cbr to allow the
scene detection to be checked once per superframe (on the base layer),
using the full resolution sources.

If scene change is detected: check for re-encoding due to
large overshoot for all spatial layers withing the superframe.

Add speed feature to control the re-encode step.
Keep the re-encode step on for now.

Small change in nonrd_pickmode to remove the possible skip of golden
reference for SVC, when the high_source_sad is set for the superframe.

Change only affects SVC encoding with screen-content mode enabled.

Change-Id: If4cfb52cb0dd0f0fce1c4214fa8b413f8f803d56