In the Gadget structure, set the GadgetType field to GTYP_PROPGADGET and
place the address of the PropInfo structure in the SpecialInfo field.

When using AUTOKNOB, the GadgetRender field must point to an Image
structure. The Image need not be initialized when using AUTOKNOB, but the
structure must be provided. These Image structures may not be shared
between gadgets, each must have its own.

To use application imagery for the knob, set GadgetRender to point to an
initialized Image structure. If the knob highlighting is done by alternate
image (GFLG_GADGHIMAGE), the alternate image must be the same size and
type as the normal knob image.