libvpx

SVC sample encoder: Change some settings/update.

Change settings for 1 pass CBR.
And only use SET_SVC control(s) if there is at least 1 layer (spatial or temporal).
This allows sample encoder to also work for 1 layer case.

Change-Id: I5b0a33c25afb2f24a3a8aa4ec8ade9afc87cd702