Starts a new frame: drops this frame's events and resize or move records, zeroes the mouse deltas
and clears the START and END flags while keeping DOWN. update_window_events calls it first.
input_button_states[Key_Code.SPACEBAR] = .DOWN | .START;
input_per_frame_event_and_flag_update();
assert(input_button_states[Key_Code.SPACEBAR] == .DOWN);input_per_frame_event_and_flag_update :: ()