Enforce VideoEncoderConfig.num_temporal_layers >= 1.

This change clarifies the semantics of this field:
  unset: Depends on context.
  == 0: Invalid.
  == 1: No temporal layering.
  >= 2: Temporal layering.

We should try to remove the wrapping optional later.

Bug: webrtc:11297
Change-Id: Id765f2dc1d31a4ba3cd424978ac6054cd60152ea
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/166528
Commit-Queue: Rasmus Brandt <brandtr@webrtc.org>
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30336}
diff --git a/api/video_codecs/video_encoder_config.cc b/api/video_codecs/video_encoder_config.cc
index 2b1adc0..6efcbf2 100644
--- a/api/video_codecs/video_encoder_config.cc
+++ b/api/video_codecs/video_encoder_config.cc
@@ -24,6 +24,7 @@
       max_bitrate_bps(-1),
       scale_resolution_down_by(-1.),
       max_qp(-1),
+      num_temporal_layers(absl::nullopt),
       active(true) {}
 VideoStream::VideoStream(const VideoStream& other) = default;
 
@@ -39,7 +40,7 @@
   ss << ", target_bitrate_bps:" << target_bitrate_bps;
   ss << ", max_bitrate_bps:" << max_bitrate_bps;
   ss << ", max_qp: " << max_qp;
-  ss << ", num_temporal_layers: " << num_temporal_layers.value_or(0);
+  ss << ", num_temporal_layers: " << num_temporal_layers.value_or(1);
   ss << ", bitrate_priority: " << bitrate_priority.value_or(0);
   ss << ", active: " << active;