EXT_TX introduces some new symbols to be decoded.
The encoder counts how many times these are used.
In multithreaded mode, the counts from the worker threads
need to be accumulated into the main thread.

This change means that VP10/VPxEncoderThreadTest now works
with more choices of cpu-used and number of passes.

Change-Id: Ibe7e6a3c58145265f4ead155ff98fb4cb37c3513