MClsStencil [Colour] Clear a stencil to colour 0 or the specified colour

Does the same as MCls but on the stencil of the currently used chunky Bitmap. The
actual value placed into the stencil /represents/ the colour you specify as if
all pixels in the graphic were of that 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.