CopyMask is a new version of the CopyBits procedure; it transfers a
bit image from the source bitmap to the destination bitmap only where
the corresponding bit of the mask rectangle is a 1. (Note that
the mask is specified as a rectangle instead of as a handle to a region.) It can be used along with CalcMask to implement the lasso copy as in MacPaint; it╒s also useful for drawing icons. CopyMask doesn╒t check for overlap between the source and destina