Periodic intra refresh

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.