[x265] Main12 partially functional

[Steve Borho]

Thanks to Min's hard work the Main12 profiles are now functional if you
stick with just the C code (either build with ENABLE_ASSEMBLY=OFF, or
run with --no-asm)

I'm working on a patch to ffmpeg that will enable feeding 12bit inputs
into our encoder (hopefully your source video is 12bits per channel or
higher)

known issues:

1. the asm testbench reports many primitives are wrong for Main12
   (mostly constant or dynamic range issues)
2. there are probably primitives that are passing tests but are still
   broken (because the test bench is not always catching dynamic range
   issues)
3. Main12 needs unique lambda tables, just as Main10's differ from Main.
   Until this gets resolved, Main12 compression efficiency will
   suffer greatly.
4. There are likely other places where the code assumes HIGH_BIT_DEPTH
   means Main10, these will only be found by code review

If anyone would like to work on any of these, please respond with a
description on which area(s) you are working.

-- 
Steve Borho