MMakeStencils [FirstBitmap.w,LastBitmap.w] ; Make stencils for all/range of bitmaps

Initialises (if necessary) and generates cookies for all or a range of chunky
Bitmap objects. If the cookie doesn't yet exist it will be allocated. The stencil
will be generated based on the contents of the Bitmap's image. If the parameters
are omitted, all currently created chunky Bitmaps will have stencils generated.
The area of the Bitmap used to make a stencil can be cropped using the Bitmap's
clip window.