x86: AVX2 high bit-depth var_16x16

40->27 cycles on Haswell.
libx264 API usage example
Safety check against malicious high bit-depth input which could cause crash
Simplified libx264 API usage example
Update to the latest version of gas-preprocessor.pl from http://git.libav.org/?p=gas-preprocessor.git

Contributions by Janne Grunau, Martin Storsjo, Mans Rullgard, David Conrad, Martin Aumuller and others