vp9-svc: Fix the setting of is_key_frame.

Change on affects 1 pass CBR.
On key frame, temporal layer_id is reset to 0 for 1 pass CBR,
but since "layer" is reset, the svc.layer_context[layer].is_key_frame
was not correspondingly set properly.

Change-Id: I08f6da0a55ac7429ccfbaddfb7be14479e43543b
1 file changed