Refactor to split the 1 passs source sad computation into scene
detection (currently used for VBR and screen-content mode), and
superblock based source sad computation (used in non-rd CBR mode).

This allows the source sad computation for CBR mode to be
multi-threaded.

No change in compression.

Change-Id: I112f2918613ccbd37c1771d852606d3af18c1388