Rename pSrc2 -> pMask in mt_merge
Actually use functions::copy_c in mt_lutspa instead of just copying its code
Remove helpers.h from lut filters
Use custom struct instead of std::pair in mt_lutxyz.

Makes things a bit more obvious
Use std::deque instead of std::list for expressions

20-30% faster lut evaluation and thus startup times of complex luts
Avoid some unnecessary copying in best processor selector