Add Luminance based clipping

[mTvare6]

Requires peniko#116

SVG has support for Luminance based clipping but vello doesn't support luminance based clipping. If the PR mentioned is merged, then this would allow the vello to use that.
https://developer.mozilla.org/en-US/docs/Web/SVG/Reference/Attribute/mask-type

[LaurenzV]

In vello_cpu, you can already use Mask::new_luminance, I don't think more is needed here to implement what's required for SVG?

[mTvare6]

In vello_cpu, you can already use Mask::new_luminance, I don't think more is needed here to implement what's required for SVG?

I'm mainly concerned with classic vello as it in required in GraphiteEditor/Graphite, so we would still require a new mix mode for classic vello, don't we?

[LaurenzV]

I'm not familiar with vello classic, but that's possible, yes. Then I would suggest removing the parts from vello_cpuand just keeping the ones in vello.

[nicoburns]

In vello_cpu, you can already use Mask::new_luminance, I don't think more is needed here to implement what's required for SVG?

@LaurenzV In vello_cpu, what is the different between specifying a mask using the mask paramter to a layer vs. creating an extra later and using blend modes to achieve a masking effect (which I believe is the only available approach in vello classic)? Is there an advantage (performance?) to the mask parameter, or is it just a convenience?

[LaurenzV]

I think this was discussed in the Zulip before, basically I wasn't aware you can use blending to simulate this effect, but in any case blending is super slow on the CPU, so I think having masks as an explicit special case makes sense there.

[Keavon]

I haven't followed closely as to the technical aspects of this and how it aligns with the desired approach within Vello's engineering direction, but I did want to pop in and just mention that this is a blocker for Graphite in its current form (unless we can use a workaround of some sort).