Support the mask-type property for masks
https://www.w3.org/TR/css-masking-1/#the-mask-type introduces a mask-type
property. In SVG1.1 the implicit default is mask-type: luminance
while now there is also mask-type: alpha
, which is quite convenient.
Librsvg needs to implement the mask-type
property, as it only does luminance masks right now.