Enable row-mt for SVC for real-time mode, speed >=5.

Add the controls to the sample encoders, but keep it off for now.
Add the control and enable it for the 1 pass CBR unittests.

For speed 7, 3 layer SVC, 2 threads, row-mt enabled gives about ~5% speedup.

Change-Id: Ie8e77323c17263e3e7a7b9858aec12a3a93ec0c1