vector: use triangulation instead of stencil buffers

[hajimehoshi]

Operating System

• Windows
• macOS
• Linux
• FreeBSD
• OpenBSD
• Android
• iOS
• Nintendo Switch
• PlayStation 5
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• Web Browsers

What feature would you like to be added?

By triangulating a polygon (spliting a polygon into triangles), we can resolve these issues:

• Rendering quality of edges is low (Vector anti-aliasing issue #3124): Now rendering vector.Path needs to use stencil buffers. A stencil buffer basically holds binary values and is not suitable for antialiasing edges. (By the way, MSAA would not work with concave polygons if I understand correctly.) Once we could triangulate polygons, we could apply some algorithms like Loop Blinn to render smooth edges.
• Rendering polygon with a transparent color doesn't work (vector: allow transparent colors for AppendVerticesAndIndicesForStroke #2409). If a polygon is just a set of convex polygons, we could easily use transparent colors.

Also, if trinagulation works well, we can simplify Ebitengine implementations by removing stencil buffers.

Why is this needed?

No response

[hajimehoshi]

I feel like this is a little higher prioarity in order to deprecate ebiten.FillRule and stencil buffers inside.

[hajimehoshi]

https://github.com/raverie-us/raverie-engine/blob/main/Code/Foundation/Geometry/Triangulator.cpp seems a very good reference for Siedel algorithm.