Win32: common code is not DirectX specific
OpenGL.h: protect double-inclusion
Win32: include headers where needed
Win32 Vout: remove unnecessary includes
Glwin32: move affinity to the glwin32 structure
Win32 vout: Remove unused sys_t member
Win32 vout: update comments
Win32: cleanup the events.c codebase

No functionnal change
DirectDraw: remove unused include
glwin32: remove unneeded includes and defines
WinGDI: remove useless includes