vp9-svc: Fixes to make SVC work with VBR

Prior to this CL SVC with VBR mode was broken.
Fixes made here to make VBR rate control work for SVC.
Rename is_one_pass_cbr_svc() --> is_one_pass_svc(),
as it can be used now for both CBR and VBR.

Added rate targetting unittest for (2SL, 3TL).

Bug: chromium:1375111
Change-Id: I5a62ffe7fbea29dc5949c88a284768386b1907a9
9 files changed