api: add zero-latency tune target

This just disables lookahead and B frames at the moment.
primitives: fix compile warning exposed when ASM and instrincs are both disabled
common: disable MSVC warning that is exposed when compiling without primitives
cpu: move ASM fallback functions out of vec-primitives.cpp

This fixes link errors when ASM and vector primitives are both disabled
api: give structs the same name as their typedef

This allows them to be forward-decl'd.
Merge with stable
common: fix int typecast to operate on results of float expression