Generalize SimulcastEncoderAdapter, use for H264 & VP8.

* Move SimulcastEncoderAdapter out under modules/video_coding
* Move SimulcastRateAllocator back out to modules/video_coding/utility
* Move TemporalLayers and ScreenshareLayers to modules/video_coding/utility
* Move any VP8 specific code - such as temporal layer bitrate budgeting -
  under codec type dependent conditionals.
* Plumb the simulcast index for H264 in the codec specific and RTP format data structures.

TBR=sprang@webrtc.org,stefan@webrtc.org,titovartem@webrtc.org

Bug: webrtc:5840
Change-Id: I2d3b130622dd7ceec5528f3ab6c46f109e6bafb8
Reviewed-on: https://webrtc-review.googlesource.com/84743
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23715}
diff --git a/AUTHORS b/AUTHORS
index 694e98e..2b09878 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -61,6 +61,7 @@
 Hans Knoechel <hans@hans-knoechel.de>
 Korniltsev Anatoly <korniltsev.anatoly@gmail.com>
 Todd Wong <todd.wong.ndq@gmail.com>
+Sergio Garcia Murillo <sergio.garcia.murillo@gmail.com>
 Maxim Pavlov <pavllovmax@gmail.com>
 Yusuke Suzuki <utatane.tea@gmail.com>
 Piasy Xu <xz4215@gmail.com>
@@ -89,3 +90,5 @@
 Wire Swiss GmbH <*@wire.com>
 Miguel Paris <mparisdiaz@gmail.com>
 Vewd Software AS <*@vewd.com>
+Highfive, Inc. <*@highfive.com>
+CoSMo Software Consulting, Pte Ltd <*@cosmosoftware.io>
\ No newline at end of file