Addressing edge case where a layer stopped before bitrate could be
measured. Purely bit rate based change deduction missed this as
the before and after did not have bit rates.
Use available layers to look for changes, especially currently
forwarding layer going away.
Also, simplifying bits. Only in the optimal allocation path,
these things are required. When congested, bitrate is always needed.
So, for optimal path, just look at available layer changes and adjust.
Don't need to look for bitrate based layer changes. Clean up that code.
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