Support mask-image

[glennw]

There is (basic) support in webrender for this now, so we will need to add parser support (if not already done for Stylo) and then pass it through via layout to display lists.

[glennw]

I've already broken mask-image support once (oops), so if we can get this supported in Servo we can enable the WPT tests related to it :)

[kvark]

I've looked a bit into this and would be able to implement this, unless someone is eager enough to rush it :)

[jdm]

Making some notes in case I find someone to tackle this:

• The parsing code exists in https://dxr.mozilla.org/servo/rev/1dfdd0bab82b2356c6fb15d0d0e898a972983ba1/components/style/properties/longhand/svg.mako.rs#194 and probably only needs the products="gecko" bit removed.
• After that, it's a matter of treating it similar to the filter CSS property - converting from CSS values into WebRender values
• We also need to provide real values as the third argument to ClippingRegion::to_clip_region, so we will want a member storing some kind of mask representation

This is where my understanding gets hazy. ClippingRegions are part of BaseFlow, but a flow can be made of multiple elements, and mask-image applies to a single element.

[nox]

mask-image is still Gecko-only.