MCls [Colour] Clear a bitmap to colour 0 or the specified colour

Performs a clearscreen on the currently used main chunky Bitmap. If specified, it
will be cleared to the colour you choose, otherwise will be cleared to 0 (usually
black). The area of the Bitmap affected by the operation can be limited by use of
a clip window and the clipping feature in the Bitmap to be used. See MDrawingMode
for a decription of the drawing modes that are available for MCls.