Re: mask: luminance or alpha

Brian Birtles:
> I've checked in my changes to the masking section including an issue
> about the default value.
>
>    https://svgwg.org/svg2-draft/masking.html#mask-property
>
> The whole section is pretty draft so feel free to hack away at it.

Looks great!  I can't remember if we discussed the mask property 
referencing paint server elements previously, but I like how that is 
handled.  I think the <ul> in 14.4.2.3 should have an extra item to 
mention paint server referencing explicitly.