Bug 970691 - Part 1: Add timestamp to fake video. r=jesup

Update YuvStamper utility. Add a CRC32 to the encoded
payload and have the decode method us this to verify reception.
Wrap encoded values across multiple lines in the frame buffer
when necessary. Use YuvStamper to encode a timestamp in each fake video frame.
Extract the value in VideoConduit to calculate the video latency
and add this to a running average latency when enabled via config.
Bug 970691 - Part2: Restore digit stamping function to YuvStamper. r=jesup

Refactor digit writing method to use the new internals. Allows digit string
to wrap through multiple lines in a small frame.