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.  If the drawing mode is
set to MReMapMode or MSimpleReMapMode with the MDrawingMode command,
MColourMode will be used instead as it is illogical to try to perform remapping
to the stencil.