Adds better initial conditions for block size encoding CDFs

Also, makes the split16 CDF dependent on whether 4x4 is present.
Fixing shebang statements in ab_compare scripts

Signed-off-by: Nathan E. Egge <negge@dgql.org>
Adds some context to the block size range symbol

Probability depends on the size of the smallest neighbouring block
Per-plane context for the skip flags

Saves ~2% on the first frames of park_joy. Not much difference on akiyo
Jointly codes the noref flags on 8x8 and 16x16

Saves ~2% on the first P-frames of parkjoy, little impact on akiyo
Addresses comments from issues 346, 347, 349.

Except initialization of the probs to increment >> 2 which I'll look into
separately.