Function            MUpdateScreen - define screen region for updating

  Syntax              void MUpdateScreen(int left, int top, int right,
                                         int bottom);

  Prototype in        mousehk.h

  Remarks             defines a region of the screen to be updated, from
                      the coordinates (left,top) to (right,bottom).

  Return value        nothing.

  Note                The mouse cursor is hidden during updating and must
                      be explicitly turned on again (via MCursorOn()) to
                      be visible.

  See also            MCursorOn()

  Example             see demomous.c