MBoxStencil Xpos.w,Ypos.w,Xpos2.w,Ypos2.w[,Colour] Draw an unfilled box in a stencil [to specified colour]

Does the same as MBox but draws the unfilled box to the stencil of the currently
used chunky Bitmap. The actual value drawn to the stencil /represents/ the
currently used ink colour or the specified colour.