[Impeller] SDF convex path prototype - #183961

Prototypes a solution for #183960

Work in Impeller is ongoing to develop SDF shaders for various primitives such as Circle, RoundRect, etc. These specific solutions will not work for arbitrary paths. This PR is a prototype for using the existing ShaderGeometry code to produce geometry meshes hinted with SDF parameters to render antialiased versions of convex paths.

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [AI contribution guidelines] and understand my responsibilities, or I am not using AI tools.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• I followed the [breaking change policy] and added [Data Driven Fixes] where supported.
• All existing and new tests are passing.

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