MSMaskScroll X1.w,Y1.w,Width.w,Height.w,X2.w,Y2.w[,SourceBitmapNum.w] ; Stencil-Copy bm 2 bm and st 2 st

Stencil-MaskScrolls an area within the currently used chunky Bitmap, or using a
different source Bitmap if specified. Width should be a multiple of 4. If
SScrollCut is Off, a normal `SMaskBlit' will be performed in which the source
Bitmap's graphic will be mask-copied to the destination Bitmap's graphic (with
masking), and the source Bitmap's stencil is copied to the destination Bitmap's
stencil (with masking). If SScrollCut is On, the destination Bitmap's stencil
will be used to protect areas of the destination Bitmap's graphic, instead of
copying the source stencil into the destination stencil, effectively scrolling the
source image `behind' the protected parts of the destination and yet with masking
to cause any areas in the source graphic of Colour 0 to be used as transparent.
Both source and destination Bitmaps should have stencils.