Correctly set number of reference buffers in H264 encoder

iNumRefFrame specifies total number of reference buffers to allocate.
For N temporal layers we need at least (N - 1) buffers to store last
encoded frames of all reference temporal layers.

There is no API in OpenH254 encoder to specify exact set of references
to be used to prediction of a given frame. Encoder can theoretically
use all available references.

Note that there is logic in OpenH264 which overrides iNumRefFrame to
max(iNumRefFrame, N - 1): https://source.chromium.org/chromium/chromium/src/+/main:third_party/openh264/src/codec/encoder/core/src/au_set.cpp;drc=8e90a2775c5b9448324fe8fef11d177cb65f36cc;l=122.
I.e., this change has no real effect. It only makes setup more clear.

Bug: none
Change-Id: If4b4970007e1cc55d8f052ea05213ab2e89a878f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/225480
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Sergey Silkin <ssilkin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34445}

modules/video_coding/utility/simulcast_test_fixture_impl.cc

@@ -590,6 +590,7 @@ void SimulcastTestFixtureImpl::SwitchingToOneStream(int width, int height) {
     settings_.VP8()->numberOfTemporalLayers = 1;
     temporal_layer_profile = kDefaultTemporalLayerProfile;
   } else {
+    settings_.H264()->numberOfTemporalLayers = 1;
     temporal_layer_profile = kNoTemporalLayerProfile;
   }
   settings_.maxBitrate = 100;