ENH: Removing inclusion of windows.h by top VTK headers.  This results in a substantial compile time reduction (~70%) on Windows compilers.  It also avoids forcing windows name mangling on user code.  Preprocessor work-arounds for this mangling has been added for VTK methods.  A new vtkWindows.h has been created for VTK code that really needs the windows API.  A vtkOpenGL.h has been created for VTK code that needs GL/gl.h because this header requires windows.h to be included first on some compilers.  This addresses bug #836.