Add a frame generator that allows scrolling over a larger still image, for use with new screen sharing quality tests.
Also add support for this in the loopback tests.

BUG=
R=pbos@webrtc.org

Review URL: https://codereview.webrtc.org/1256713004 .

Cr-Original-Commit-Position: refs/heads/master@{#9652}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 89934133988feaf14fb88f5258f727232a283d0f
3 files changed