glutGetModifiers returns the modifier key state when certain callbacks were generated.
glutGetModifiers returns the modifier key state at the time the input event for a keyboard, special, or mouse callback is generated. This routine may only be called while a keyboard, special, or mouse callback is being handled. The window system is permitted to intercept window system defined modifier key strokes or mouse buttons, in which case, no GLUT callback will be generated. This interception will be independent of use of glutGetModifiers.