In screenshare mode, suppress VP8 bitrate overshoot and increase quality

This change includes several improvements:

* VP8 configured with new rate control
* Detection of frame dropping, with qp bump for next frame
* Increased target and TL0 bitrates
* Reworked rate control (TL allocation) in screenshare_layers

A note on performance: PSNR and SSIM is expected to get slightly worse with this cl. Frame drops and delays should however improve.

BUG=4171
R=pbos@webrtc.org, stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1193513006.

Cr-Commit-Position: refs/heads/master@{#9495}

webrtc/modules/video_coding/main/source/generic_encoder.h

@@ -149,6 +149,7 @@ private:
     const bool internal_source_;
     mutable rtc::CriticalSection rates_lock_;
     VideoRotation rotation_;
+    bool is_screenshare_;
 }; // end of VCMGenericEncoder class
 
 }  // namespace webrtc