Fixing #116: PaintComposite with an unbounded source

[justvanrossum]

This fixes #116.

Fix each backend

• Skia
• CoreGraphics
• Cairo
• SVG (Doesn't support composite mode at all at the moment, still shouldn't crash)

Note this should be generalized to the non-solid paint canvas methods.

Needs a test font + test glyph.

• Add small subset of Noto-COLRv1 as test case

[anthrotype]

I don't know if you want to go that route, but in nanoemoji.colr_to_svg (where we translate from COLR => SVG) we special case a src-in PaintComposite with transparent black PaintSolid background to mean SVG's group-level opacity, since that pattern is so common:

https://github.com/googlefonts/nanoemoji/blob/2ffdd23faad153c5cec763b6e210befc9d704cd1/src/nanoemoji/colr_to_svg.py#L283-L305

Also, I wonder, since your SVGSurface already knows its viewBox, if you could just use that when you find an unbounded paint, as if the unbounded paint were implicitly bounded by a rectangle the size of the viewbox.. though I am not sure when/whether SVG renderers would clip shapes outside the viewbox rectangle, I think it depends on the overflow property but IIRC its default changes depending on whether it's applied on the root <svg> element of a standalone document or not..

[justvanrossum]

The bigger problem is that the SVG backend doesn't implement composition to begin with. Once that's there, your second suggestion may indeed help to overcome the problem.