rtc: Add zero bitrate exit to encoder

For SVC encoding, it is common to set the
top spatial layer bitrate to 0 on the fly,
to dynamically disable layers. In such a case
we should drop the layer frame, even if the
frame-droppper is off.

Change-Id: I301e0edf86d85b4a0116a9174edb47b1646674fe

av1/encoder/encoder.c

diff --git a/av1/encoder/encoder.c b/av1/encoder/encoder.c
index 4d2fe37..750e97c 100644
--- a/av1/encoder/encoder.c
+++ b/av1/encoder/encoder.c
@@ -3392,7 +3392,7 @@
   // Never drop on key frame.
   if (has_no_stats_stage(cpi) && oxcf->rc_cfg.mode == AOM_CBR &&
       current_frame->frame_type != KEY_FRAME) {
-    if (av1_rc_drop_frame(cpi)) {
+    if (cpi->oxcf.rc_cfg.target_bandwidth == 0 || av1_rc_drop_frame(cpi)) {
       av1_setup_frame_size(cpi);
       av1_rc_postencode_update_drop_frame(cpi);
       release_scaled_references(cpi);