An alpha soft mask's vector art is whitened so its luminance is its
alpha, but images kept their colour: cairo (and so Inkscape) draws a
mask as an image whose alpha is in its own SMask, often black pixels,
so the mask came in empty. While reading an alpha mask's art the
importer now reads images white with their own alpha (under their own
cache key, without the JPEG and CMYK shortcuts), and stencil masks
white; a luminosity mask nested inside reads them as they are.
Fixes#1550