This updates the horizontal and vertical sliders to the values you provide
(HPos and VPos), and redraws the gadget.
This is useful when you have a group of sliders to update and all you need to
change is their slider positions (as opposed to all the parameters, which
ModifyProp lets you do).
INPUTS
Window
Pointer to the window the PropInfo structure resides in.
GadgetID
Short integer ID of the Gadget.
HPos
Short integer position for horizontal slider between 0 and 65535.
VPos
Short integer position for vertical slider between 0 and 65535.