Don't include the lazy precompiled.h everywhere, only what's
required for the compilation unit.
platform.h needs to be included instead to provide all essential
defines and types.
All includes use the relative path to the neo or the game
specific root.
Move all idlib related includes from idlib/Lib.h to precompiled.h.
precompiled.h still exists for the MFC stuff in tools/.
Add some missing header guards.
Use the CVar s_numberOfSpeakers for that, its already stored
in there.
Add the same (!= 2 && !=6) check as in the audio backends since
the mixer is limited to these 2 values.
