MMakeStencil BitmapNumber.w ; Make a stencil for a bitmap

Initialises (if necessary) and generates a stencil for a specific chunky Bitmap
object. If the stencil doesn't yet exist it will be allocated. The stencil will
be generated based on the contents of the Bitmap's image. The area of the Bitmap
used to make a stencil can be cropped using the Bitmap's clip window.