MBoxFStencil Xpos.w,Ypos.w,Xpos2.w,Ypos2.w[,Colour] Draw a filled box in a stencil [to specified colour]

Does the same as MBoxF but draws the filled 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.