On Windows, we use the Win32 for Unicode conversion. On OSX, we don't
even need to convert (and probably not on recent Linux distros either,
but they all have iconv anyway :) )

confere posix_memalign documentation

This allows using pl_Yield() from video outputs and friends, on
condition that pl_Yield() return value be checked for NULLity.

Should fix #1667.