Control metric to be queried with this message. This can be either of the
following:
VMA_ITEMSIZE
If this message attribute is set,
the width and height of each item (in pixels) are returned in the usItemWidth
and usItemHeight parameters, respectively.
VMA_ITEMSPACING
If this message attribute is set, the horizontal
and vertical spacing between items (in pixels) is returned in the usHorzItemSpacing
parameter and in the usVertItemSpacing parameter, respectively.