slicetype: increase baseline bframe bias by 10%

Our estimateFrameCost() function is not yet checking bidir candidates (because
bidir search in the main encoder needs to be replaced first) and since B frame
estimates cannot use intra modes either this tends to make B frame scores
relatively higher than what x264 finds.  So our default bias needs to be a bit
higher in order for the lookahead to select B paths.
frameencoder: move threadMain() into cpp file