Use SDL to set video modes, get a GL context and detect the
amount of VRAM.
As with the Linux GLimp implementation, sys_videoRam can be used
in case the SDL detection fails.
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.
