Fix artefacts when using multiscale masks + parallel deconvolution

When combining multiscale clean with auto-masking and parallel deconvolution, sources near the boundary may be cleaned by two subimages, because multiscale ignores the single-scale mask when scale-dependent masks are set. This is fixed by making sure the scale-dependent masks are intersected by the boundary mask before multiscale is called.

This fixes an issue reported by Reinout. In his data set, an artefact appeared at a more or less random position in the image.