Add first_spatial_layer_to_encode to SVC.

Use the existing VP9_SET_SVC control to set the
first spatial layer to encode.

Since we loop over all spatial layers inside the encoder, the
setting of spatial_layer_id via VP9_SET_SVC has no relevance.
Use it instead to set the first_spatial_layer_to_encode,
which allows an application to skip encoding lower layer(s).

Change only affects the 1 pass CBR SVC.

Change-Id: I5d63ab713c3e250fdf42c637f38d5ec8f60cd1fb