Expects: AX    0001H
Returns: none
Info: This unhides the mouse pointer.  It actually increments an
internal counter used by the mouse support to determine when to
show the pointer.

That counter starts as -1 (after an INT 33H 0000H or 0021H
reset).  This call increments it to 0.

Whenever the counter is 0, the mouse pointer is displayed and
tracked on-screen.  When the counter is 0, subsequent Show
Pointer calls are ignored.  Calls to INT 33H 0002H (hide pointer)
decrement the counter.

This logic relieves programs of the burden of global tracking of
the hidden/displayed state.  A subroutine may always use
INT 33H 0001H at the beginning and INT 33H 0002H at the end,
without affecting the shown/hidden state of the calling routine.

This function also resets the "exclusion area" set via

- -

INT 33H 0001H: Show Mouse Pointer