[cairo] Output SVG with fill and alpha stroke, but without inline images ?

[Stuart Axon]

I've been trying to generate an SVG using SVGSurface with a fill and an alpha stroke, 
with OPERATOR_SOURCE to ensure the stroke draws over the fill, but this seemsto generate <image> tags in the SVG.
Toy example and output + XML:
https://gist.github.com/stuaxo/b525a47d33f5f3074b1be7c693f6b6e6
Is there any way to render fill + stroke (with alpha) and only output vector data ?This seems like a common operation, but whatever I try outputs SVGs full of <image>s.

For Shoebot, this has meant our SVGs become very very large, enough to eat all your ram if you try and open them in Inkscape.
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