can be enabled/disabled with the r_useStencilOpSeparate for comparisons
(like Z-Fail, this doesn't really seem to make a difference on my main
machine, neither on my RPi4)
Partly based on Pat Raynor's Code:
2933cb5545/neo/renderer/draw_stencilshadow.cpp
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.
