Fix multithreaded encoding for 1 MB wide frame

Thread synchronization was not correct when frame width was 1 MB.
Number of allocated encoding threads is limited by the sync_range.
There is no point having more because each thread lags sync_range MBs
behind the thread processing the row above.

http://code.google.com/p/webm/issues/detail?id=302

Change-Id: Icaf67a883beecc5ebf2f11e9be47b6997fdf6f26