MMakeStencil [FirstBitmap.w[,LastBitmap.w]] ; Make a stencil for a bitmap, range of bitmaps, or all bitmaps

Initialises (if necessary) and generates stencils for a specific bitmap, a range
of bitmaps, or all bitmaps. 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. If
the parameters are omitted, all currently created chunky Bitmaps will have
stencils generated. The area of a Bitmap used to make a stencil can be cropped
using the Bitmap's clip window. If a second parameter is specified you can make
stencils for a range of bitmaps.