This method is a special version of im_erase for images that support
im_drawframe.  it asks an image to erase itself as if it were confined
(scaled, clipped, etc.) to a rectangular bounds.  The return value for
this method is not explicitly defined.

This method uses a custom message structure:

struct impErase /* NOTE: This is a subset of impDraw */
    {
        ULONG           MethodID;   /* IM_ERASEFRAME */
        struct RastPort *imp_RPort; /* The image's RastPort */
        struct
        {
            WORD X;      /* X and Y offset relative to the */
            WORD Y;      /* image's IA_Left and IA_Top attributes */
        } imp_Offset;

        struct
        {
            WORD Width;  /* scale, clip, restrict, etc. to these bounds */
            WORD Height;
        } imp_Dimensions;
    };

The imageclass dispatcher handles an im_eraseframe message as if it was an
IM_ERASE message, ignoring the bounds.  See the imageclass description for
im_erase for more details.