refine YUV and coeff buffer
preset: disable rectangular and AMP partitions at medium preset

This gives a considerable speed-up (50-70%) at the default preset for not much
loss in compression efficiency (2-3%). AMP is rarely useful, at non-RDO presets
we were only considering AMP merge anyway, and rect can be approximated by a
split and two merges.
cmake: 32-bit macho compiles need _ prefixes on exported functions
ratecontrol: improve visual quality and bitrate savings in ABR.

Try to prevent ABR over-compensation after I frames by amortizing the cost over
the next few frames;

Improve ABR quality with frame parallelism - enable frame parallelism only after first
few P frames to prevent excessive qp fluctuations.

Fix initial I frame qp. when cu tree is enabled, the qp decided arbitrarily is
too low. This causes a huge qp spike in immediate P frames.Tuned cplxrSum
for intial I frame so that a more appropriate qp is chosen.
rc: cleanups for clarity