Seams between fills in canvas renderer

[Herschel]

Due to the method of anti-aliasing in most SVG renderers, shared edges will appear to have seams in the canvas render backend. "Conflation artifacts" are the technical term for these.

Example (Ruffle canvas backend on top, Flash on bottom):

(Also see #24)

The standard advice to avoid this is to overlap the paths, e.g. an outline fill should appear on top of the fill underneath it. But Flash almost always generates non-overlapping paths to avoid overdraw.

I'm not sure there is an easy fix for this beyond disabling AA completely with SVG properties like shape-rendering: crispEdges. But I think this will be kind of moot, as WebGL rendering (via tessellation and/or Pathfinder) will be the long-term goal over canvas.

[jrmuizel]

https://github.com/jrmuizel/full-scene-rasterizer is a possible solution to this problem. There's a non-full-scene rust port of this C++ code here: https://github.com/jrmuizel/raqote/blob/master/src/rasterizer.rs

[Herschel]

For canvas renderer specifically, I was wondering if we can "fix" this by bumping out one side of any shared edges by some small threshold. When processing an SWF shape in ShapeConverter, we know whenever an edge is shared, so it might be fairly easy to split one side of the edge into two segments and bump out the midpoint. This might look bad if a shape is alphaed out, but worth a test.