Expects: AX    000bH
                                                                  
Returns: CX    Horizontal motion since last call (>0=right; <0=left)
DX    Vertical motion since last call (>0=down; <0=upward)
                                                                  
Info: This obtains the distance the mouse has been moved since the most
recent call to this function.  The returned values are in units
called mickeys.  To convert to pixels, use INT 33H 001bH to query
the current mickeys-to-pixel ratios).

Note that CX and DX return as signed values, with negative
numbers being a distance upward and to the left and positive
values being motion downward and to the right.

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INT 33H 000bH: Query Motion Distance