when changing curve twist I assumed 2D curves would not need dir or quat values set, however these are used when getting a path from a curve.
also added interp_v3_v3v3v3v3
* Fix remaining issues before conversion.
* Inline various vector functions, currently enabled for all platforms.
I expect this to work in GCC/MSVC at least, if other platforms don't
support it, #ifdef's can be added.
