MMaskScroll X1.w,Y1.w,Width.w,Height.w,X2.w,Y2.w[,SourceBitmapNum.w] ; Copy bitmap graphic with stencil-cut

Performs a masked copy of an area from within one chunky Bitmap to somewhere in
another chunky Bitmap. The currently used main Bitmap is the destination, and is
also the source unless you specify otherwise. Width should be a multiple of 4 and
the entire operation should fit within the dimensions of the chunky Bitmaps and
will not be cropped. Any chunky Bitmaps used should have a stencil created for
them and this will be used to perform a masked copy. Any holes in the stencil, ie
any areas of colour 0 in the source Bitmap, will be transparent.