Investigate drawing clear brushes in the opaque pass

[kvark]

See #3252 (comment) for some context.

[mstange]

And yes, I agree that ideally we'd want to the clear brushes to be done in the opaque pass, just with the color masked out (and only alpha written).

I believe you need to write to both RGB and A, otherwise you'll have invalid premultiplied values. If A is 0, RGB also have to be zero for valid premultiplied RGBA.