Periodic intra refresh
Uses SEI recovery points, a moving vertical "bar" of intra blocks, and motion vector restrictions to eliminate keyframes. Attempt to hide the visual appearance of the intra bar when --no-psy isn't set. Enabled with --intra-refresh. The refresh interval is controlled using keyint, but won't exceed the number of macroblock columns in the frame. Greatly benefits low-latency streaming by making it possible to achieve constant framesize without intra-only encoding. Combined with slice-max size for one slice per packet, tests suggest effective resiliance against packet loss as high as 25%. x264 is now the best free software low-latency video encoder in the world. Accordingly, change the API to add b_keyframe to the parameters present in output pictures. Calling applications should check this to see if a frame is seekable, not the frame type. Also make x264's motion estimation strictly abide by horizontal MV range limits in order for PIR to work. Also fix a major bug in sliced-threads VBV handling. Also change "auto" threads for sliced threads to "cores" instead of "1.5*cores" after performance testing. Also simplify ratecontrol's checking of first pass options. Also some minor tweaks to row-based VBV that should improve VBV accuracy on small frames.
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