Some initial keyframe-only bw adaptivity handling code.

Currently handles only coefficient probability updates, which occur
on all frame types. All mode probabilities (which occur only for
interframes) are not yet handled. I haven't yet confirmed that the
output of these updates are identical to what libvpx produces.
Simplify intra mode edge checks a little (same complexity, less code).
Invert array order for mode_conv[] (less code, same complexity).
Semi-rewrite sub-exponential diff probability update reading.
Use right-shift instead of division in inv_recenter_nonneg().

The numbers are never negative, so this prevents the compiler from
introducing a + (v < 0) before doing the rightshift.
vp9dsp/WIP: idct32_1d