The test scenes can now supply their own optional base (background)
color. In the with_winit example, we also allow the user to provide a
base color as a CLI option. The precedence is as follows:
1. Use the color from the CLI options, if any.
2. Otherwise use the scene provided base color, if any.
3. Otherwise default to black.
Added two scenes that demonstrate conflation artifacts as described in
https://github.com/linebender/vello/issues/49. The first scene
demonstrates adjacent triangles and rects that belong to the same path
and use opposite winding.
The second scene demonstrates strokes with overlapping square caps
(these strokes are currently expressed as rects painted with the NonZero
fill rule).
